Cloyster
Type: Water / Ice
Base Stats: 50 HP / 95 Atk / 180 Def / 85 SpA / 45 SpD / 70 Spe
Ability: Shell Armor / Skill Link
Cloyster is a Pokemon with rather unremarkable stats other than its stand-out 180 base Defense. Due to its great physical bulk, Cloyster can be a great user of Spikes and an excellent lead for any offensive team in need of Spikes to support their sweepers. Against many leads, Cloyster can setup 2 or even 3 layers of Spikes. Cloyster has access to Surf, which, in combination with its decent Special Attack stat, allows it to defeat many other opposing leads before they get the chance to setup their own hazards, like Metagross, Gliscor, Hippowdon, Bronzong, Skarmory, and Forretress, among others. The combination of Surf and Ice Shard can take down any opposing frail leads, such as Azelf and Aerodactyl. If facing off against something that could potentially sweep Cloyster's teammates, Cloyster can simply explode and KO any Pokemon it wants without any worry, as Cloyster need not survive past the first few turns. Overall, any offensive team appreciates the Spikes that Cloyster provides, and Cloyster has all of the tools it needs to be an excellent lead.
Regirock
Type: Rock
Base Stats: 80 HP / 100 Atk / 200 Def / 50 SpA / 100 SpD / 50 Spe
Ability: Clear Body
Blessed with amazing defenses and a decent Attack stat, Regirock can function as a check to some of the top sweepers in the OU tier, including Heatran and Zapdos. Very few special attackers can do much of anything to Regirock once sand is up, and with Tyranitar being so prevalent, this situation is very common. Regirock can even beat Pokemon with super effective moves, such as Gyarados, Dragonite, Lucario, Scizor, and Celebi, all due to Regirock's phenomenal Defense stat and Special Defense stat under sand. Regirock is usually sporting a Curse set, with Rest providing recovery and pseudo-EdgeQuake with Rock Slide. It does take multiple turns of setup for Regirock to be actually threatening offensively, but the Defense boosts are invaluable against such foes as Gyarados, Lucario, and Scizor. Regirock is not something often seen, but if sand is up, breaking it is not going to be an easy task.
I will take Weezing, Claydol, and Cresselia because why not.
EDIT:
Weezing
Type: Poison
Base Stats: 65 HP / 90 Atk / 120 Def / 85 SpA / 70 SpD / 60 Spe
Ability: Levitate
Weezing is quite the strange Pokemon, boasting a high Defense stat but low HP stat, as well as a Poison typing but immunity to Ground-type moves due to Levitate. Its movepool is just as strange, including options non-characteristic to a Poison-type like Fire Blast and Thunderbolt. However, all of these traits combine to create an excellent check to many top-tier physical attackers like Gyarados, Lucario, and Tyranitar. Due to Levitate, it can switch-in on powerful Ground-type attacks from the likes of Mamoswine and Hippowdon. Although its Special Defense stat is disappointing, it doesn't take very much damage from any of Celebi's attacks not named Psychic, an attack most Celebi don't carry, boasting an immunity to Earth Power and resistance to Grass-type attacks, as well as not being affected all that much by Thunder Wave and being immune to Toxic, with Hidden Power Fire not doing much damage either. Weezing has Fire Blast to take on Steel-types like Skarmory and Forretress, something less experienced players may not even expect. Thunderbolt does solid damage to Gyarados. which it checks exceptionally well due to its great physical bulk. To top it all off, Weezing has recovery in Pain Split, which is great for wearing down opposing threats and is even better due to Weezing's low HP stat, turning that downside into a selling point. Access to Will-O-Wisp only furthers Weezing's ability to check physical attackers, with Taunt and even Explosion sometimes making an appearance. When Weezing comes out on the field, there are very few physical attackers that can beat it, and it occupies a solid niche for this reason.
EDIT2:
Claydol
Type: Ground / Psychic
Base Stats: 60 HP / 70 Atk / 105 Def / 70 SpA / 120 Def / 75 Spe
Ability: Levitate
Claydol is among those niche Pokemon with limited use, but excel at their use, and Claydol's is the coveted combination of Stealth Rock and Rapid Spin. Claydol makes a decent spinner because it resists Stealth Rock and is immune to Spikes and Toxic Spikes thanks to Levitate, as well as boasting resistances to Stone Edge, immunity to Earthquake, immunity to Electric-type moves, and resistance to Fighting-type moves, all combined with excellent defenses and a few offensive options that hit a few notable Pokemon super effectively. Even with such low offensive stats, Ground-type STAB is still Ground-type STAB, and that means Pokemon such as Metagross and Lucario will lose quite a chunk of their health to such attacks. Ice Beam threatens Flygon and Dragonite, both of which Claydol can check rather adequately. Claydol can even utilize Explosion and take out an opposing threat as it survives an attack. While very niche, Claydol's access to Stealth Rock and Rapid Spin, as well as its unique set of resistances, particularly its rare resistance to the famed EdgeQuake combo, Claydol can function as a decent spinner and Stealth Rock setter for any team in need of such a Pokemon that needs both roles to be filled by one Pokemon.
Cresselia
Type: Psychic
Base Stats: 120 HP / 70 Atk / 120 Def / 75 SpA / 130 SpD / 85 Spe
Ability: Levitate
Cresselia is blessed with the best defenses of almost every Pokemon usable in OU, allowing it to wall many powerful attackers. Although it does struggle against the very common Tyranitar and Scizor, as well as failing to do much lasting damage against anything with much bulk, it has enough bulk to repeatedly take hits and setup screens, whether it be just Reflect or both. Cresselia can use Thunder Wave to cripple the many faster Pokemon that fail to do any meaningful damage to it. Its STAB Psychic gives it super effective coverage on Infernape and other Fighting-types, as well as on Gengar. Ice Beam can take down Dragon-types, which Cresselia easily walls. Cresselia even has access to recovery in Moonlight, though it is crippled in sand and hail. Cresselia may also sport Charge Beam along with RestTalk and Ice Beam, with its great bulk allowing it to boost its Special Attack and have pseudo-BoltBeam coverage. A Calm Mind set can be used to take on many special sweepers and sometimes even pull off a sweep of its own. Cresselia has a niche as a user of screens due to access to Lunar Dance, which allows Cresselia to sacrifice itself and fully revive a teammate, acting even before entry hazards, so Pokemon in range of being KOed by entry hazards can have a second chance at life. Against a decent amount of OU threats, Cresselia can shine, walling them to no tomorrow, and although it falters against Tyranitar and Scizor, two very common Pokemon, it can do well against many other threats and is a wall to be reckoned with.
I am done ^_^ reserving Blastoise and Cradily
EDIT3:
lol wtf is Blastoise doing as a defensive threat it's an anti-lead
Blastoise
Type: Water
Base Stats: 79 HP / 83 Atk / 100 Def / 85 SpA / 105 SpD / 78 Spe
Ability: Torrent
Blastoise has one niche and one niche only: it can function as an excellent anti-lead. Due to having access to Rapid Spin and STAB priority in Aqua Jet, Blastoise can take down many opposing leads. Hydro Pump followed by Aqua Jet will take down the majority of leads, including Aerodactyl, Azelf, Metagross, Infernape, Jirachi, Swampert, and Machamp. Ice Beam gives a 100% accurate hit on Gliscor and hits Dragonite as well as Flygon. Blastoise's excellent defenses allow it to take a hit or two from these leads and more. If the opposing lead happens to be a hazard lead, Blastoise will likely outlast it and be able to spin away the hazards, making them a non-factor for its teammates throughout the match.
Cradily should probably be offensive as well (it's a Curse sweeper, it just happens to be quite bulky) although it is a bit more in the middle because it walls and sweeps at basically the same time.
Cradily
Type: Rock / Grass
Base Stats: 86 HP / 81 Atk / 97 Def / 81 SpA / 107 SpD / 43 Spe
Ability: Suction Cups
Cradily is an excellent Curse sweeper for a number of reasons. For one, its already stellar Special Defense stat gets boosted by 50% under sand, a very common weather condition due to the prevalence of Tyranitar. Also, it has a low Speed stat and usable Attack stat to abuse, as well as coveted access to Recover, giving it a niche as the only viable user of Curse that has reliable recovery. Cradily has all of the coverage it needs in pseudo-EdgeQuake, which very few Pokemon resist, all of which can be easily removed by the right teammates. Due to its excellent defensive stats and Recover, Cradily can often obtain multiple boosts to makes it attacks hit hard and make it nigh-impenetrable as a wall. RestTalk mono-attacker variants are also threatening, as in that case even Toxic cannot stop Cradily, making playing stall against it all but a fools' game. Although it has a weakness to the very common Fighting-type attacks, that is its only negative attribute. With Suction Cups, it can't even be phazed, making one of the most common detriments to setup sweepers a non-issue. Cradily also has access to Swords Dance, which gives it the ability to boost its Attack quickly, and Cradily can dish out quite a bit of pain after it has boosted up. Trick is the best option to beat Cradily; it limits the many-tentacled beast to one move, which turns it from a monster to a failed fossil recovery. Cradily is a Pokemon to watch out for, as, although it is rarely seen, it could potentially set up to a point where it walls your entire team and OHKOes them as well.
Those are done too :DDD I will take the last unreserved Pokemon in Mesprit and Qwilfish. I will then check through all of my previous submissions and make sure all of the QC changes are implemented.
EDIT4:
Mesprit
Type: Psychic
Base Stats: 80 HP / 105 Atk / 105 Def / 105 SpA / 105 SpD / 80 Spe
Ability: Levitate
Due to its excellent defenses and stellar Special Attack stat, Mesprit can function as a utility Pokemon that can KO many top-tier threats, including Gyarados, Dragonite, Infernape, and Flygon. BoltBeam and STAB Psychic give it all of the coverage it needs, with very few Pokemon resisting the combination of the three. Mesprit also has access to Thunder Wave, which allows it to remedy its only below-average stat, its Speed. U-turn allows Mesprit to scout Tyranitar or Scizor and do solid damage in return thanks to Mesprit's 105 base Attack. It can even setup Stealth Rock, finding room for it when its teammates cannot. Mesprit even has access to Trick Room, which allows it to function as both a standalone sweeper that no longer suffers for its below-average Speed stat and a supporter for teammates on the slower side. Mesprit can even don a Choice Scarf and become an excellent revenge killer, with U-turn keeping momentum on Mesprit's side. Levitate gives it a Ground-type immunity, granting it pretty much free revenge kills against any fast Pokemon with a Ground-type attack, such as Flygon and Aerodactyl. Overall, Mesprit's stellar balanced stats and solid movepool allow it to function as one of the best anti-metagame Pokemon.
EDIT5:
Qwilfish
Type: Water / Poison
Base Stats: 65 HP / 95 Atk / 75 Def / 55 SpA / 55 SpD / 85 Spe
Abilities: Swift Swim
Qwilfish is a very simple threat, the turns it is alive being a very simple process. First, a teammate sets up Rain Dance, preferable with Damp Rock to maximize the amount of turns Qwilfish has to sweep, in turn boosting Waterfall by an additional 50% and doubling Qwilfish's Speed thanks to Swift Swim. The next move is to switch Qwilfish into a resisted attack, hopefully a choiced one, setup Swords Dance, and proceed to have 4 turns to KO Pokemon on the opposing team. On the last turn of rain, although it could potentially be any turn, Qwilfish explodes, meaning something will die. This simple process is, in reality, a fairly easy sweep once the possible opposing Tyranitar or Abomasnow is removed. Other than that, Qwilfish has an excellent utility in removing Toxic Spikes, which rain teams hate, but there isn't much else. If you run into Qwilfish under rain and you are not prepared for it, you should be prepared for it to sweep you, simple as that.
Woohoo I am done with this stuff! I will now go through and make sure all of my previous entries are fixed with the QC changes. Yay!
EDIT6: I have finished updating all of my submissions with QC stuff, and deleted Jolteon to eliminate any sort of confusion. Yay done!