Clefable
Abilities: 3 (Cute Charm, Magic Guard, Unaware)
Recovery: Softboiled, Moonlight, Wish
Boosting: Calm Mind, Belly Drum, Cosmic Power, Work Up, Amnesia
Priority: None
Hazards: Stealth Rock
Hazard Removal: None
Overall Attack Diversity: Very High
People Have already talked enough about Clefable. It has two amazing abilities, a massive movepool and Clefable itself is already a top tier OU pick. Out of all these Pokemon, Clefable is the most complete package.
Keldeo
Abilities: 1 (Justified)
Recovery: None
Boosting: Calm Mind, Swords Dance, Work Up
Priority: Aqua Jet
Hazards: None
Hazard Removal: None
Overall Attack Diversity: Average
Not the best choice out there, despite its power in OU. It is worth noting that you have strong boosting options for both sides, and its single ability might find more use on its followers than itself. Essentially, your entire team dissuades the use of Knock Off, which is neat.
Tornadus-T
Abilities: 1 (Regenerator)
Recovery: None
Boosting: Bulk Up, Agility
Priority: None
Hazards: None
Hazard Removal: None
Overall Attack Diversity: High
Great ability and attack diversity for all-out offensive teams, but you lack a lot key aspects to give your followers. Regenerator does mitigate the lack of recovery, however.
Alakazam(-Mega)
Abilities: 3 (Synchronize, Inner Focus, Magic Guard ->
Trace)
Recovery: Recover
Boosting: Calm Mind, Barrier
Priority: None
Hazards: None
Hazard Removal: None
Overall Attack Diversity: Average
Unless you really want to use Alakazam, Clefable is probably a better overall choice. Alakazam and its Mega fill the fast and strong Attacker better than Clefable, however, so its still usable.
Azumarill
Abilities: 2 (Thick Fat,
Huge Power, Sap Sipper)
Recovery: None
Boosting: Belly Drum, Work Up, Amnesia, Defense Curl
Priority: Aqua Jet
Hazards: None
Hazard Removal: None
Overall Attack Diversity: Low
Not worth using without Huge Power. Its two other abilities are solid, but its movepool leaves a lot to be desired.
Bisharp
Abilities: 3 (Defiant, Inner Focus, Pressure)
Recovery: None
Boosting: Swords Dance, Rock Polish, Iron Defense, Hone Claws
Priority: Sucker Punch
Hazards: Stealth Rock
Hazard Removal: None
Overall Attack Diversity: Low
Actually has pretty decent options, with boosting, priority, Stealth Rock and Defiant to dissuade hazard control on any member of the team. Your special movepool is fairly limited, but a Hyper Offense team is still very possible.
Charizard-Mega(-X/Y)
Abilities: 2 (Blaze, Solar Power ->
Tough Claws,
Drought)
Recovery: Roost
Boosting: Dragon Dance, Swords Dance, Hone Claws, Defense Curl
Priority: Quick Attack
Hazards: None
Hazard Removal: Defog
Overall Attack Diversity: Low
If you're using Charizard, it would be for the Mega Charizard Y Drought + Solar Power combo. You also provide Defog to mitigate your own hazard weakness, team-wide recovery and strong physical boosts. Mega Charizard X is still usable, but your followers abilities won't be especially useful.
Diancie-Mega
Abilities: 1 (Clear Body ->
Magic Bounce)
Recovery: None
Boosting: Calm Mind, Rock Polish, Harden
Priority: None
Hazards: Stealth Rock
Hazard Removal: None
Overall Attack Diversity: Low
Mega Diancie is alright, but diversity is all-around pretty lacking. You do give Stealth Rock and hazard-blocking with your own Magic Bounce, but you won't be able to pass that on, so everyone is stuck with Clear Body.
Excadrill
Abilities: 3 (Sand Rush, Sand Force, Mold Breaker)
Recovery: None
Boosting: Swords Dance, Hone Claws, Iron Defense
Priority: None
Hazards: Stealth Rock
Hazard Removal: Rapid Spin
Overall Attack Diversity: Average
Aside from recovery, priority and special boosting, Excadrill is a fairly complete package. Mold Breaker Stealth Rock + Rapid Spin is pretty sweet, and you even have a few weird moves like Sludge Bomb to pass on. If you stick a Sandstorm-setter on your team, you strongly benefit from two of your abilities.
Ferrothorn
Abilities: 2 (Iron Barbs, Anticipation)
Recovery: None
Boosting: Swords Dance, Curse, Rock Polish, Iron Defense
Priority: None
Hazards: Stealth Rock, Spikes
Hazard Removal: None
Overall Attack Diversity: Average
You could do worse than Ferrothorn. Basically, your whole team screws over physical attackers, has two hazards and a usable miscellaneous movepool (why does it get Thunderbolt?) You also mitigate the lack of recovery with Leech Seed.
Garchomp(-Mega)
Abilities: 2 (Sand Veil, Rough Skin ->
Sand Force)
Recovery: None
Boosting: Swords Dance, Hone Claws
Priority: None
Hazards: Stealth Rock
Hazard Removal: None
Overall Attack Diversity: Average
Another contact damage abuser! Garchomp has stronger offensive options, but Ferrothron has more utility. The choice is up to you.
Heatran
Abilities: 1 (Flash Fire,
Flame Body)
Recovery: None
Boosting: None
Priority: None
Hazards: Stealth Rock
Hazard Removal: None
Overall Attack Diversity: Average
Heatran noticeably has a few holes in its movepool. As a Flash Fire-passer, Arcanine is probably a better choice, but Stealth Rock is always nice, and you gain a powerful trapping move in Magma Storm.
Hoopa-U
Abilities: 1 (Magician)
Recovery: None
Boosting: Nasty Plot, Calm Mind
Priority: None
Hazards: None
Hazard Removal: None
Overall Attack Diversity: High
If you're using Hoopa-U, its for a powerful Hyper Offense squad. If you want to do anything but attack, Hoopa-U is not for you. Do note that no followers can use Hyperspace Fury.
Landorus-T
Abilities: 1 (Intimidate)
Recovery: None
Boosting: Swords Dance, Calm Mind, Bulk Up, Rock Polish
Priority: None
Hazards: Stealth Rock
Hazard Removal: None
Overall Attack Diversity: High
Not only is a full team of Intimidate users really annoying, but your attack movepool spans a gambit of different types, special and physical. Your boosting options cover any possible use, and you even have Stealth Rock.
Lopunny-Mega
Abilities: 3 (Cute Charm, Klutz, Limber ->
Scrappy)
Recovery: None
Boosting: Work Up, Cosmic Power, Agility
Priority: Fake Out, Quick Attack
Hazards: None
Hazard Removal: None
Overall Attack Diversity: High
You have a few perks, like double priority and a huge movepool only a Normal type could have, but none of your abilities are especially noteworthy. If you want to use the Mega itself, you can make a serviceable team around it, but there are better options.
Latios(-Mega)
Abilities: 1 (Levitate)
Recovery: Roost, Recover
Boosting: Calm Mind, Dragon Dance
Priority: None
Hazards: None
Hazard Removal: Defog
Overall Attack Diversity: High
Latios is really solid. Levitate is great on certain types and you give recovery, Defog and physical/special boosting options to your team. Your attack movepool is also very wide.
Manaphy
Abilities: 1 (Hydration)
Recovery: None
Boosting: Tail Glow, Calm Mind
Priority: None
Hazards: None
Hazard Removal: None
Overall Attack Diversity: Average
Manaphy has one major use, and that's Tail Glow. Abilities, utility and physical options are all very lacking, but if you want to use something like Tail Glow Alakazam or Gengar, this is your own choice outside of Volbeat.
Metagross-Mega
Abilities: 2 (Clear Body, Light Metal ->
Tough Claws)
Recovery: None
Boosting: Hone Claws, Agility, Rock Polish, Iron Defense
Priority: Bullet Punch
Hazards: Stealth Rock
Hazard Removal: None
Overall Attack Diversity: High
You get a bit of everything with Mega Metagross. As a one-time ban candidate, it might be worth using even if, as a leader, it doesn't do anything truly unique.
Sableye-Mega
Abilities: 3 (Keen Eye, Stall, Prankster ->
Magic Bounce)
Recovery: Recover, Moonlight
Boosting: Calm Mind, Nasty Plot, Hone Claws
Priority: Sucker Punch, Fake Out, Shadow Sneak, Feint
Hazards: None
Hazard Removal: None
Overall Attack Diversity: Average
True hell. People have talked about Sableye's potential for defensive teams, but its numerous options for boosting and priority allow offensive Pokemon to succeed as well, creating solid Balance teams. Your coverage isn't the best, but you definitely have perks that other leaders lack.
Scizor(-Mega)
Abilities: 3 (Swarm, Technician, Light Metal)
Recovery: Roost
Boosting: Swords Dance, Agility, Iron Defense
Priority: Bullet Punch, Vacuum Wave, Quick Attack, Feint
Hazards: None
Hazard Removal: Defog
Overall Attack Diversity: Low
Scizor lacks coverage, but provides nearly everything else. Probably the best choice for a Technician team, as you can use Hidden Power can supplement your lack of special movepool. Boosted Vacuum Wave is also neat.
Talonflame
Abilities: 2 (Flame Body, Gale Wings)
Recovery: Roost
Boosting: Swords Dance, Bulk Up, Hone Claws, Agility
Priority: Quick Attack, [Gale Wings]
Hazards: None
Hazard Removal: None
Overall Attack Diversity: Low
People have discussed Talonflame already. What it lacks in diversity, it supplements with the most powerful priority options in the game. Special attackers are hard to pull off, and you can't remove hazards for your multiple Flying types, but its still a solid leader.
Thundurus
Abilities: 2 (Prankster, Defiant)
Recovery: None
Boosting: Nasty Plot, Bulk Up, Agility, Charge
Priority: None
Hazards: None
Hazard Removal: None
Overall Attack Diversity: High
While Thundurus appears to lack much of what makes a good leader, it has serious perks for offensive teams. Prankster and Defiant are both solid abilities, you have great boost options and your coverage spans a good number of types.
Weavile
Abilities: 2 (Pressure, Pickpocket)
Recovery: None
Boosting: Swords Dance, Nasty Plot
Priority: Ice Shard, Fake Out, Quick Attack, Feint
Hazards: None
Hazard Removal: None
Overall Attack Diversity: Low
Weavile is very give-and-take. Your boosting and priority options are some of the best around, but your abilities, utility and coverage are all below average. Weavile itself is a great Pokemon, but there are better leader options, especially when Weavile potentially gains a lot as a follower.