(Mega) Alakazam
Weaknesses: Bug, Dark, Ghost
Gets Tail Glow, Quiver Dance and Dark Void. Pretty neat
Mega Altaria
Weaknesses: Rock, Ice, Electric (Swablu), Dragon (Base form), Fairy, Poison, Steel
Looks like it gets a lot, but really the only useful moves it gets are Shift Gear and possibly Spacial Rend. Still a solid Pokémon though
Azumarill
Weaknesses: Grass, Electric, Poison, Steel (Azurill)
Bolt Strike and Power Whip can help it break through opposing bulky Water types, while Shift Gear can help it sweep late game. Good stuff
Bisharp
Weaknesses: Fighting, Ground, Fire
Close Combat and Precipice Blades/Earthquake are good coverage as always, V-create/Sacred Fire could help too
Breloom
Weaknesses: Fire, Ice, Flying, Poison, Fairy, Psychic, Bug (Shroomish)
V-create is as always awesome coverage, Technician Icicle Spear could be interesting too. U-turn could be helpful.
Celebi
Weaknesses: Fire, Ice, Bug, Poison, Flying, Dark, Ghost
Tail Glow makes Celebi insanely powerful, Fire and Ice coverage in Blue Flare and Ice Beam respectively is always nice and if it does find itself walled it can just take it out with Dark Void or scout with Parting Shot. A lot of good stuff
Chansey
Weaknesses: Fighting
Eh...Nothing useful
Charizard
Weaknesses: Electric, Rock, Ground (Charmeleon), Water, Dragon (Mega X)
Doesn't really get anything especially useful. Crabhammer could deal with Diancie and Heatran and gets a Tough Claws boost unlike Earthquake
Clefable
Weaknesses: Steel, Poison
Eh...again gets virtually nothing useful
Conkeldurr
Weaknesses: Psychic, Flying, Fairy
Play Rough could be used to wreck Mega Sableye, but I doubt that'll be common in this metagame. Can also set up its own Trick Room now
Mega Diancie
Weaknesses: Ground, Water, Grass, Steel
Good amount of stuff, Earthquake/Thousand Arrows for extra coverage and good utility moves in Spore, Spiky Shield and King's Shield
Dragonite
Weaknesses: Ice, Rock, Dragon, Fairy
Like Altaria it looks like it gets a lot but in reality not much of it is actually useful. You could try a special set with Geomancy I guess
Excadrill
Weaknesses: Ground, Water, Fighting, Fire, Ice (Drilbur), Grass (Drilbur)
Good amount of stuff. STAB Thousand Arrows helps you break through airborne Pokémon. Spore and Spiky Shield are nice utility moves
Ferrothorn
Weaknesses: Fire, Fighting
...nope, nothing useful. Ferrothorn's not a good burn spreader since Fire types do switch into it
Garchomp
Weaknesses: Ice, Dragon, Fairy
Gets the Dragon Dance it always wanted...and that's about it.
Gardevoir
Weaknesses: Ghost, Steel, Poison
Eh...nothing especially useful.
Gengar
Weaknesses: Dark, Ghost, Psychic, Ground
Nasty Plot, Dark Void and Parting Shot are all moves Gengar can use well. Doesn't get much from Ghost, Psychic and Ground though
Gliscor
Weaknesses: Water, Ice
Haze I guess...
(Mega) Gyarados
Weaknesses: Electric, Rock, Grass (Magikarp), Fighting (Mega forme), Fairy (Mega forme), Bug (Mega forme)
Bolt Strike could be useful against opposing bulky Waters, gets reliable recovery in Heal Order (or Synthesis if not running Mega) and a scouting move in U-turn. Close Combat could be used to wreck Ferrothorn
Heatran
Weaknesses: Water, Ground, Fighting
Could use Steam Eruption to hit opposing Fire types if you dislike Earth Power, while Secret Sword lets you beat Chansey. You could even set up Spikes
Hippowdon
Weaknesses: Grass, Ice, Water
Spiky Shield and Spore are nice utility moves. Icicle Crash could be used for extra coverage if you dislike Stone Edge's accuracy.
Hoopa-Alt
Weaknesses: Bug, Fairy, Dark (Confined), Ghost (Confined)
Geomancy makes it a very scary sweeper, Sucker Punch helps bypass lacklustre Speed, Parting Shot could be used to scout and Dark Void could put a would be counter out of commission. Tail Glow is an option as well, although its Speed is pretty lacklustre. Scary stuff
Jirachi
Weaknesses: Dark, Ghost, Fire, Ground
Nasty Plot, Dark Void and Parting Shot are useful as ever. Sacred Fire with Serene Grace is good for a guaranteed burn, while Earth Power and Blue Flare are nice coverage moves to hit opposing Steel types. You can even set up Spikes if you want. Lots of good stuff
Keldeo
Weaknesses: Grass, Electric, Psychic, Flying, Fairy
Synthesis for reliable recovery, Energy Ball and Thunderbolt for opposing Water types, and a great boosting move in Geomancy. If Mega Venusaur presents a problem, you also get Oblivion Wing. Damn that's scary
Klefki
Weaknesses: Fire, Ground
Prankster Will-O-Wisp...and that's it
Kyurem-Black
Weaknesses: Fighting, Steel, Rock, Dragon, Fairy
Close Combat, Shift Gear, Diamond Storm and Geomancy. Pretty good
Landorus-T
Weaknesses: Water, Ice
Eh...you get Icicle Crash if you really hate Gliscor. Nothing else notable
Latias
Weaknesses: Bug, Dark, Ghost, Dragon, Ice, Fairy
God damn that's a lot of good stuff! Tail Glow, Dark Void, Parting Shot, Spacial Rend and Geomancy! Geomancy + Stored Power = death.
Latios
See Latias
Mega Lopunny
Weaknesses: Fighting, Flying (Mega), Psychic (Mega), Fairy (Mega)
You only really get a more reliable Fighting STAB in Close Combat from this. Bulk Up could be used to boost I guess
Magnezone
Weaknesses: Ground, Fire, Fighting
Earth Power, Blue Flare and Aura Sphere/Secret Sword/Focus Blast enable Magnezone to trap and kill just about every defensive Steel type.
Manaphy
Weaknesses: Grass, Electric
Reliable recovery in Synthesis is nice, and you get Volt Switch for scouting. Other than that though, it doesn't get much. Spore isn't very useful for Manaphy as it tends to draw in bulky Grass types
Mega Manectric
Weaknesses: Ground
Earth Power is some extra coverage I guess...
Mega Metagross
Weaknesses: Fire, Ground, Dark, Ghost
Tough Claws V-create is very scary. You could also shut down a would be counter with Dark Void, or use Parting Shot to scout. You could even try a bulky booster set with Curse and Gyro Ball (I don't recommend this), and you get Tough Claws Knock Off and Drill Run.
Mew
Weaknesses: Dark, Ghost, Bug
Parting Shot and Dark Void are as ever useful utility moves. Tail Glow or Quiver Dance combined with Mew's movepool could be extremely threatening
Raikou
Weaknesses: Ground
See Mega Manectric
Rotom-Wash
Weaknesses: Grass, Ground, Dark (base), Ghost (base), Rock (Oven), Water (Oven), Bug (Mow), Ice (Mow), Fire (Mow), Poison (Mow), Fighting (Freezer)
Everything you could ever possibly want. Reliable recovery in Heal Order, better STAB in Steam Eruption and all the coverage you could ever need, boosting in Quiver Dance and Tail Glow, and 2 sleep inducing moves (although Dark Void's better for Rotom-W)
Mega Sableye
Weaknesses: Fairy
Eh...nothing particularly useful
Scizor
Weaknesses: Fire, Rock (Scyther), Ice (Scyther), Electric (Syther), Flying (Scyther)
Extra coverage in V-create and Bolt Strike, Technician Rock Blast and Icicle Spear could be interesting too
Serperior
Weaknesses: Fire, Ice, Flying, Bug, Poison
Gone is the formerly crap movepool. Contrary V-create and Overheat are incredible. Also gets Ice Beam to hit Dragon types and opposing Grass types. U-turn could be used to scout
Skarmory
Weaknesses: Fire, Electric
Nothing really useful
(Mega) Slowbro
Weaknesses: Grass, Electric, Dark, Bug, Ghost
Giga Drain and Thunderbolt could be useful to hit opposing bulky Water types, Quiver Dance is a nice alternative to Calm Mind and Regenerator with Parting Shot/U-turn/Volt Switch is great for scouting. Dark Void's useful as ever. Good stuff
Starmie
Weaknesses: Grass, Electric, Dark, Bug, Ghost
Tail Glow with 115 base Speed makes for a frightening special sweeper. Dark Void is more useful than Spore as only Insomnia/Magic Bounce Pokémon are immune
Sylveon
Weaknesses: Steel, Poison, Fighting (Eevee)
Secret Sword and Aura Sphere are useful coveage moves. You could even try a Coil set with Shift Gear or Coil (I don't recommend this)
Talonflame
Weaknesses: Water, Rock, Electric, Ice (Fletchling)
Crabhammer and Bolt Strike are useful coverage moves
Thundurus
Weaknesses: Rock, Ice
Gets the BoltBeam combo, and Prankster Haze
Tornadus-Therian
Weaknesses: Rock, Ice, Electric
Say hello to the best Rain abuser with perfectly accurate Hurricanes and Thunders.
(Mega) Tyranitar
Weaknesses: Fighting, Water, Ground, Grass, Bug, Steel, Fairy, Ice (Pupitar)
Close Combat and Thousand Waves for coverage, reliable recovery in Heal Order, scouting in U-turn, a boosting move in Shift Gear, reliable Sleep inducing in Spore...Just in case you thought Tyranitar's movepool was too small. That's scary
Mega Venusaur
Weaknesses: Fire, Ice, Flying, Psychic
You could try a bulky Calm Mind set. You also get a better recovery move in Roost and Defog if you care
Weavile
Weaknesses: Fighting, Steel, Rock, Fire, Bug, Fairy
Close Combat and V-create help with bulky Steel types, U-turn could be used for scouting. Good stuff
Zapdos
Weaknesses: Rock, Ice
Ice Beam for BoltBeam, but nothing else really notable