Use this (I know that Heatran and Keldeo were already posted, but those are different sets):
Heatran @ Air Balloon / Life Orb
Trait: Flash Fire
EVS: 56 HP / 252 SAtk / 200 Spd (An spread of 252 HP / 252 SAtk / 4 SDef is also viable; always use 4 HP / 252 SAtk / 252 Spd if you use Flame Charge)
Timid Nature (+Spd, -Atk) (Modest should be used if you run a bulky spread)
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Fire Blast
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Earth Power
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Hidden Power [Ice]
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Flame Charge /
Will-O-Wisp /
Toxic /
Stealth Rock /
Taunt
Role: This is actually hard to define. This Heatran is somewhat of a supportive attacker, since it's more offensive, but has ways to support the team. But with Flame Charge, it could instead be classified as a special sweeper. I would define Heatran's hole as a
Tank, Support, Late-game sweeper (if it runs Flame Charge)
What it does: Heatran is peharps one of the best, if not the best offensive supporter for teams. It has a lot of applications that are extremely useful for all teams. First, it acts as a check to Dragons that lack Fighting- or Water-type attacks. Second, it defeat almost all Steel-types with its STAB Fire Blast, and even opposing Heatran, the only Steel-type that laugh at Fire attacks, are defeated by Earth Power, as long as they don't carry Air Balloon themselves. Third, Heatran is the best counter to sun sweepers, and if you manage to get rid of Dugtrio, Heatran can easily annoy a sun team to the hell. Finally, it can have a different role, depending of its last move.
Fire Blast is an obvious option for STAB. It defeats all common Steel-types, except opposing Heatran, and also defeats the ubiquitous Grass- and Bug-types, that are actually very common on the metagame. Earth Power is there mainly because otherwise, opposing Heatran would wall him. But Heatran is also useful on other situations against Rock-types, or against Magnezone on rain, or when you cannot afford a miss with Fire Blast against Lucario or extremely weakened Steel-types. Hidden Power Ice is there to defeat Dragon-types, Gliscor, and Landorus. This is the move that allows Heatran to check Dragon-types. With that EV spread, Heatran can outspeed max speed Adamant Dragonite, and after Stealth Rock, OHKO him. Hidden Power Ice also OHKOes Garchomp, as long as it doesn't hold Yache Berry, and can OHKO any Salamence that lacks Brick Break.
The last move has a lot of useful options. Flame Charge is the main one, as with it, Heatran can be a late-game sweeper. Also, Flame Charge allows Heatran to beat Dugtrio if it opts to not use Air Balloon. After it uses Flame Charge on the switch, Heatran can use HP Ice (or Fire Blast, but HP Ice is more accurate and OHKOes Dugtrio anyway) to OHKO Dugtrio, since not only Heatran outspeeds Dugtrio after a Flame Charge, but Flame Charge breaks Dugtrio's Focus Sash. However, there are other moves that Heatran can also run. Will-O-Wisp and Toxic are two viable status moves that Heatran can use. The former surprises Fighting-types (except Conkeldurr) and Gyarados, expecting to be able to defeat Heatran. They are crippled for the rest of match if they switch on this move. Toxic, on other hand, is more effective against bulky Water-types, as they would be able to stall out Will-O-Wisp if they pack reliable recovery. As no Steel-type is going to switch on Heatran, Heatran is almost guaranteed to badly poison an opponent. Toxic is also the only way which Heatran is going to defeat Volcarona 1x1, as apart from a Fire Blast critical hit, despite being able to wall Volcarona, Heatran can't actually do much back. Apart from these three options, there is a fourth. If there are no other Pokémon on your team to setup Stealth Rock, Heatran is always there to do this job. However, there are many other, more reliable users of Stealth Rock. Only use this if there are absolutely NO OTHER viable users. Taunt is the final viable option. With it, Heatran can beat Forretress without letting him setup hazards. Heatran can also surprise Chansey/Blissey switch-ins, as well as disrupt CM Jirachi on rain, and surprise many other setup sweepers. However, Will-O-Wisp is a more effective way to surprise physical setup sweepers, while Toxic is more effective against special ones. However, Taunt is able to catch both in one move, and Toxic is not effective against the aforementioned CM Jirachi.
The EVs let Heatran outspeed Adamant Dragonite (even with 252 Speed EVs) and OHKO him with HP Ice after Stealth Rock damage. As Heatran will be using HP Ice, there is no point to run max Speed, since Heatran will never be able to outpace opposing Heatran that don't run HP Ice, and there's nothing notable that you outspeed by running max Speed. So, the leftover EVs go to HP, to give Heatran a little more bulk. If you use Flame Charge, however, always use 252 Speed EVs. Another viable EV spread is a bulky one, with 252 HP / 252 SAtk / 4 SDef and a Modest nature. This lets Heatran act as a more efficient tank, and Heatran can now actually take Dragon-type attacks better. For example, Heatran only has a small chance to be 2HKOed by +1 Adamant Dragonite's Outrage after Stealth Rock, and without Stealth Rock, it's never 2HKOed. However, Heatran is now outsped by a lot of things, the most notable ones being defensive Gliscor and Adamant Dragonite. Look the benefits of each EV spread, and choose wisely.
As for the item choices, the main one is Air Balloon, because it's that item, combined with the move Hidden Power Ice, what lets Heatran check Dragon-types. Heatran can also avoid Earthquake/Earth Power on other situations, like against Hippowdon and Celebi. It can also reliably defeat opposing Heatran that pack Earth Power, as long as they don't have Air Balloon themselves. Heatran actually acts as a great check to Hippowdon if it chooses to use Air Balloon, and even a counter, if Hippowdon doesn't have Ice Fang, as Heatran can avoid an Earthquake and 2HKO with Fire Blast if Hippowdon doesn't invest on Special Defense (even without Life Orb, Fire Blast is a 2HKO). A Life Orb is also a viable option if you choose to run Flame Charge on the last slot, as Heatran will pack much more of a punch that is needeed to sweep. However, Heatran can no longer effectively check Dragon-types, and even Air Balloon is still a viable option with Flame Charge.
Good Teammates: Ninetales is a good partner because Heatran has its Fire Blast boosted by the sun. Heatran can also take care of some Dragon-types that are problematic for sun teams. With Earth Power, it can also get rid of opposing Heatran, which are also problematic for sun, especially if they lack Dugtrio.
This is not to say that Heatran is not effective outside sun teams. Sand teams can also use Heatran to a great extent, since they actually have problems dealing with sun sweepers that have boosted their stats. Heatran can also deal with Skarmory, a big headache for most sandstorm sweepers. Weatherless teams also benefit from the fact that Heatran is protecting them from being swept by sun teams.
A good partner is Jellicent because with Air Balloon, your opponent is forced to use Water- and Fighting-type attacks to deal with Heatran, but Jellicent at same time discourages that your opponent use these attacks, since they are free switch-ins to Heatran. Jellicent can also spinblock in case that Heatran decides to use Stealth Rock, and the opponent wants to spin it away. Celebi is peharps the best partner to Heatran that exists. Not only Celebi resists all Heatran's weakness and vice-versa, but Celebi can also deal with the bulky Water-types that Heatran can't touch, while Heatran deals with the Dragon-types that Celebi has problems with. The only type that the two don't resist thogeter is Rock. Starmie also resists Water and Fighting, and can get rid of most Water-types using Electric-type attacks. Latias and Latios are also two other good partners that resist Fighting- and Water-type attacks.
What counters it: Chansey and Blissey are the best counters to Heatran. They shrug off anything that Heatran throws at them. Even if Heatran packs Toxic, they have Natural Cure to get rid of poison. Even if Heatran surprises them with Taunt, they can simply use Seismic Toss to slowy kill Heatran. However, if the pink blobs decide to use Toxic + Flamethrower/Ice Beam, Heatran actually walls them. Any bulky Water-type that is not weak to Ground (because of Earth Power) can effectively counter Heatran. They take little damage from Earth Power, resist Heatran's other moves, and can defeat him with Water-type attacks. Politoed is peharps the worst of them, since it can summon a rain that greatly cuts Heatran's effectiveness. However, all of them, except Hydration Vaporeon and Starmie, must avoid Toxic, and those that don't pack reliable recovery, like the aforementioned Politoed, must also avoid Will-O-Wisp. Gyarados is another Water-type that can beat Heatran, but on a offensive way, however, it must avoid Will-O-Wisp, and is weak to the Stealth Rock that Heatran is capable of laying.
Breloom is the only Grass-type that actually beats Heatran, since it is also a Fighting-type, and can OHKO with Mach Punch. Bulky versions of Heatran can even be outsped and KOed by Low Sweep. That said, Breloom must avoid Fire Blast or Will-O-Wisp on the switch, since it will be OHKOed by the former, and crippled by the latter. Also, Poison Heal versions of Breloom can actually be beaten by Heatran, as long as they aren't behind a Substitute, since they don't pack Mach Punch, and are outsped and OHKOed by Fire Blast.
Speaking of Fighting-types, they are also great checks in general, though most of them must avoid Will-O-Wisp. Conkeldurr and Keldeo are two exceptions: the former actually has its attack boosted by Will-O-Wisp (or Toxic) thanks to Guts, and the latter doesn't mind burns, as it is a specially-based attacker. However, Conkeldurr is 2HKOed by Fire Blast (even if Heatran does not use Life Orb), so it's not completely safe. Terrakion is a good check, but besides Will-O-Wisp, it is 2HKOed by Earth Power (except on sandstorm). If it manages to switch-in safely, however, Terrakion can wreck avoc. The next switch-in will certainly take a lot of damage from Choice Band-boosted Close Combat. Lucario outspeeds Heatran even with an Adamant nature, but it must avoid Fire Blast, Earth Power and Will-O-Wisp, so Lucario is hard-pressed to find a good oportunity to switch-in on Heatran.
If you manage to pop Air Balloon, Heatran gains many other checks. For example, Gliscor only needs 236 Speed EVs to outspeed and OHKO Heatran with Earthquake. Hippowdon becomes a good check, as it can survive one Fire Blast and retaliate with Earthquake. Choice Scarf Landorus can outspeed Heatran even after one Flame Charge boost, and OHKO with Earthquake or Earth Power. Gastrodon can also use Earth Power to defeat Heatran, although most Heatran are going to switch out anyway, for the fear of Scald.
If none of these options can be used, there's always one thing that can defeat Heatran: the surprise factor. For example, Scizor can OHKO Heatran with Superpower. However, Scizor must hit Heatran on the switch, as the only way which it can outspeed Heatran is if it does not run speed EVs, and Scizor does run. As long as Air Balloon has been poped, Venusaur can make a nasty and rage-inducing surprise to Heatran with Earthquake. Venusaur with Hidden Power Fighting can also surprise Heatran. Venusaur doesn't generally run those moves because it's going to lose coverage on other, more relevant threats, so rest assured as Venusaur is hardly going to use these moves. Avoid trying to check Dragonite partnered with Politoed, since it's certainly going to use Waterfall. The only time where you can check this Dragonite is after Stealth Rock, if Dragonite hasn't boosted its Speed with Dragon Dance, and it doesn't use a Jolly nature. However, many Dragonite can actually surprise Heatran with Superpower, even outside rain teams. Like Venusaur, Dragonite rarely carry those moves, so you can rest assured. Dragonite generally don't run Waterfall anyway, as it is going to be walled by Ferrothorn otherwise, and Ferrothorn is an arguably more relevant threat than Heatran (especially to rain teams). Heatran laughs at almost all moves that Celebi carry, but if it has Earth Power, then Heatran is fucked. And to a magma monster, it will be very embarrassing to lose to a little cute fairy.
Keldeo @ Choice Specs
Trait: Justified
EVS: 4 HP / 252 SAtk / 252 Spd
Timid Nature (+Spd, -Atk)
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Hydro Pump
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Secret Sword
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Hidden Power [Electric] /
Hidden Power [Ghost]
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Icy Wind /
Surf
Role: Wallbreaker, late-game sweeper
What it does: Keldeo is peharps the best user of Choice Specs on OU, and for good reason. Keldeo has the same stats of the monstrous Terrakion (only the SpA and Atk are reversed). Can't run a Fire-type Pokémon or Fire-type attack to defeat Steel-types? No problem, Keldeo is there to defeat all Steel-types that you are going to encounter. Scizor is always OHKOed by Hydro Pump, even outside rain, while Skarmory is also OHKOed after Stealth Rock damage. Secret Sword 2HKOes both Ferrothorn and Heatran, while Forretress, can't survive a Hydro Pump, even when not boosted by rain. Metagross is also another Steel-type that can't survive Hydro Pump, even when not boosted by rain. Even specially defensive Jirachi is 2HKOed by Hydro Pump, OUTSIDE rain! Keldeo has a lot of things that separate him from other Specs users, first, is its resistance to Stealth Rock. The second is Secret Sword, which bypasses Blissey and Chansey, making Keldeo the only special sweeper that can easily defeat them (without any prior setup) on the entire game. The third is a good speed (same as Terrakion) and a respectable bulk (Keldeo is not OHKOed by Starmie's Thunderbolt)
Hydro Pump is the main Keldeo's STAB. It unfortunately has a bad accuracy (same as Stone Edge, the "worst attack on the game"), but its sheer power compensates for that. The things said above just show how the last thing that you would want is to understimate Hydro Pump's power. Secret Sword is the secondary STAB, that has an amazing effect; it hits opposing Pokémon on their physical defense instead of their special defense (similar to Psyshock). This means that Keldeo can easily bypass the bane of most other special sweepers, the pink blobs! Blissey is always OHKOed, while Chansey is always 2HKOed, but has a chance to be OHKOed after Stealth Rock damage! Secret Sword is also a good attack with amazing neutral coverage, good to use in situations where you cannot miss with Hydro Pump. It's important to note that Secret Sword 2HKOes Gastrodon, which means that Keldeo can use Hidden Power Electric instead of wasting it on Hidden Power Grass. Almost all other Water-types must use Hidden Power Grass because for them, catching Gastrodon is more important than to catch Gyarados and Tentacruel.
Speaking of Hidden Power Electric, it is the main option on the third slot. It offers much-needed coverage, hitting super-effectively things that resist both of its STABs, such as Tentacruel, Gyarados, and especially Jellicent (a pokémon that is immune to both Hydro Pump and Secret Sword). Hidden Power Electric also hits most bulky Water-types slighty harder than Secret Sword, as long as they aren't specially defensive. Hidden Power Ghost is an alternative that, while not hitting Gyarados and Tentacruel for super-effective damage, catches Lati@s and Celebi, two things that resist both Keldeo's STABs AND Hidden Power Electric. It's best used on rain teams, where Hydro Pump will be boosted enough to 2HKO Gyarados and Tentacruel with it.
The last slot is reserved to Icy Wind. Sure, it's a mediocre, weak attack, but it has enough power to, when boosted by Choice Specs, 2HKO Latios, 2HKO some Latias, and OHKO Salamence and Garchomp and 2HKO Haxorus without having to resort to Hydro Pump. It can also 2HKO Dragonite (OHKO if Multiscale has been broken). It also catches Venusaur, while serving as a more accurate attack than Hydro Pump to OHKO both Landorus and Gliscor. Its side effect is nice, as even if you predict wrongly, you can possibly switch to something that can beat the opponent when its speed is lowered. However, Icy Wind can be replaced by Surf. Surf is a good alternative to use, instead of Hydro Pump, when you cannot afford a miss. It's best used on rain teams, in tandem with Hidden Power Ghost, where Surf will be used as the main attack, OHKOing or 2HKOing the aforementioned Steel-types without having to resort to Hydro Pump's bad accuracy, while Hydro Pump will be used against Pokémon that resist it, as most of them will be 2HKOed anyway due to rain.
Good Teammates: This Keldeo doesn't need specific teammates, as it has sheer power, and enough coverage to get past of most of its counters. In fact, it's the Pokémon that will want to be partnered with Keldeo, as its wallbreaking power allow many Pokémon to sweep late-game. That said, Politoed and Tyranitar stand out as the best partners to Keldeo. The former boosts Hydro Pump to insane levels, transforming Hydro Pump on an atomic bomb capable of powering through many Pokémon that resist it! The latter defeats Latios, Latias, Celebi and Jellicent, allowing Keldeo to spam Hydro Pump without much problem. Tyranitar also appreciates the fact that Keldeo defeats some of its biggest enemies, such as Scizor.
Paralysis support is nice, so that Keldeo becomes somewhat of a sweeper. On rain teams, this can be provided by Jirachi, but Celebi is also another good option. Jirachi is a good partner in general because it can defeat Tornadus-T, a problem to Keldeo if it manages to switch-in safely. Again, this Keldeo doesn't really need specific partners in general, as it can wreck avoc on every non-sun team by itself.
What counters it: This Keldeo has no true counters, as Keldeo, with the right move, can bypass all of them. Slowking, Specially defensive Jellicent and Specially defensive Celebi are the closest things to true counters that Keldeo has. Specially defensive Celebi can wall Keldeo that don't carry Hidden Power Ghost, but it's 2HKOed by that move after Stealth Rock damage. However, Keldeo has many checks that it must watch out for. Such checks are Tornadus-T, Choice Scarf Thundurus-T, Jolteon Lati@s. All of them can, if they manage to get on battle safely, OHKO Keldeo with the appropriate move. Starmie can also do this, but Thunderbolt will not OHKO a full-health Keldeo, so Starmie needs Psyshock to effectively always OHKO Keldeo. Choice Scarf Landorus and Choice Scarf Terrakion do about 80% damage to Keldeo with Earthquake (on the sandstorm) and Close Combat, so Keldeo must still watch out for sandstorm teams. Should Keldeo lose the speed tie, it's murdered by a Choice Band-boosted Close Combat from Terrakion. On sun teams, most Chlorophyll sweepers, such as Venusaur, can outspeed Keldeo if the sun is shining, and OHKO him with Grass-type attacks.