Greninja: OU, ability to run both a physical and a special set + STAB on all moves, pretty good speed, workable attacking stats.
Talonflame: OU, ability to run different sets, strongest priority in the game, pretty good speed, workable physical attack stat.
Aegislash: OU, ability to run both a physical and a special set, very nice typing, mind games to be played, incredible atk/def stat (depending on form)
Goodra: OU, ability to run both a physical and a special set, pseudo-legendary, vast movepool, incredible defensive stats and workable attacking stats
Trevenant: BL, high chance to outlive any enemy pokemon, workable movepool, workable defensive stats
Noivern: BL+/OU, ability to use multiple powerful moves, very good speed, workable sp. atk stat, not bulky enough though and only viable set is Choice Specs (1-trick pony)
Zygarde: BL+/OU, high base stat total (in comparison with non-legendaries), terrible typing, average movepool, might see play as a Dragon Dance set
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Azumarill: OU, ability to run different sets, huge power, fairy-typing makes it into a dragon slayer, decent bulk
Togekiss: OU, ability to switch into common threats, paraflinch hax, decent bulk, workable moveset, nice typing
Megas:
M-Venusaur: OU, high base atk and def stats, ability helps cover some typing disadvantages
M-Charizard X: BL+/OU, makes opponent wonder which mega stone you're holding, very decent atk and speed, best used as a Dragon Dance set, still needs to use a turn to mega evolve (= stealth rock damage)
M-Charizard Y: OU, insta-sun, very high sp. atk, very decent speed (high with timid), sadly still takes a lot from Stealth Rock
M-Blastoise: BL+/OU, arguably one of the most powerful rapid spinners, ability does limit it however
M-Alakazam: OU, incredibly fast, extremely high Sp. Atk, above average moveset, normal alakazam's magic guard protects it from Stealth Rocks, extremely fragile/should be used as a glass cannon, Trace is extremely random (can get bizarre abilities or powerful ones like Parental Bond), should be viewed as a Life Orb Alakazam
M-Kanghaskan: OU+/Uber?, best SD in game, workable bulk and speed, ability to run both a physical and a special set, arguable most easily obtainable +2 sucker punches in the game, uber ranking possibility being looked into
M-Pinsir: OU, strongest quick attack in the game?, workable stats, workable movepool, can gain moxie boost before mega evolving, one of the neatest ability in the game
M-Gyarados: OU, should be treated like a different gyarados set, nothing more
M-Aerodactyl: BL+, very nice speed, best used as taunt set, not bulky enough to sweep easily, needs to be looked into further
M-Ampharos: BL, unnessary typing and hair, relatively usely ability, terrible speed, average movepool, high sp.atk
M-Scizor: OU, very nice typing, unnessary mega evolution (choice band scizor is arguably better), can learn defog and grab momentum with u-turn, very nice stats all-over
M-Heracross: OU, skill link rock blast + pin missile + arm thrust, might consider running normal Hera over mega, ugly, very high physical stats, average speed
M-Houndoom: Similar to M-Alakazam (- Magic Guard and Trace), not best ability, can't bring out full potential unless under sun
M-Ttar: OU, very high base stat, ability to run physical, special, support sets, brings in sand through sand stream
M-Gardevoir: OU, typing turns it into a Dragon Slayer, very high special stats, workable speed and movepool
M-Mawile: OU, huge power gives it insanely high attack, very decent defensive stats, workable movepool
M-Aggron: OU, high physical attack stat and defensive stats, workable movepool, ability helps out typing issues, loses the ability to take Twaves
M-Medicham: OU, pure power gives high attack, very decent stats, workable movepool, more or less useful typing
M-Manectric: BL+/OU, linear playstyle, high speed and special attack, workable defensive stats, one-trick pony,
M-Banette: OU, access to one of the most powerful abilities in the game, high physical attack stat, average bulk
M-Absol: BL+/OU, access to Magic Bounce, high physical attack and speed, decent movepool, not to be put into same category as Espeon and Xatu because it needs to use a turn to mega-evolve before being able to bounce stuff
M-Garchomp: OU, bulkier version of regular Garchomp, loses some speed, ability needs sand to bring out full potential
M-Lucario: OU, access to one of the best abilities in the game, retains very nice typing, ability to run both a physical or a special set, very decent speed, might be a waste of a mega though (same situation as Scizor)
M-Abomasnow: OU, bulkier version of regular Abomasnow, slowest among Megas and slow among regular pokemon, very decent attack stats, ability to run both a physical and special set, access to Snow Warning