My Top 6 Pokemon (especially on our site) would come down to these, imo:
1. Mega Mawile
2. Greninja
3. Mega Altaria
4. Aegislash
5. Latias
6. Mega Lopunny
1. Mega Mawile may not have much speed, but it is capable of outspeeding a good percentage of walls in the tier and getting behind a Substitute (forcing the opponent to make a very tough decision). Being behind this Substitute guarantees that Mawile will be able to get up a swords dance (at plus 2 2HKO's Ferrothorn with SP). Even some of the bulkiest Pokemon in the game have issues with taking that guaranteed +2. (It's best Pokemon to go against would most likely be Ferrothorn as Gyro Ball only has a slight chance of breaking the Substitue).
2. Hello? It's Greninja. Protean. Move. Destruction. Life Orb.
3. Mega Altaria is good for all of the reasons listed by
@Matt_192 . This Pokemon has various sets, all of which are viableon most teams. Its Fairy/Dragon typing after Mega Evolving is excellent (only having 4 weaknesses, 7 resistances and 1 immunity). Not to mention the divers move pool that this Pokemon is given gives it excellent coverage as a Mixed attacker; getting moves like Flamethrower, Dracon Dance, Return to use alongside Pixilate for STAB. The possibilities with this Pokemon are endless.
4. Good typing, great mixed stats, excellent weakness policy user, very tricky Pokemon.
5. I know that this may come as a surprise, but I'm looking at this from a supportive standpoint. Latias is capable of Defogging hazards, dealing heavy damage with a life Orb boosted Draco Meteor/Psyshock , and can work wonders late-game with Healing Wish. That being said, I do think that late-game healing wish is a huge contributor as to why this Pokemon is on my list, seeing as I've seen it spawn tremendous comebacks for and against me.
6. Mega Lopunny is one of those Pokemon that's just really hard to play against. It's fast and not many things are too eager to face off against it (except TalonGod of course). Mega Lopunny has one of those things that few Pokemon get: a guaranteed Mega Evolution with near 0 repercussions (-things like Rocky Helmet, Rough Skin, Iron Barbs, and Joboca berry. Also Inner Focus Pokemon or other Pokemon with similar abilities.) Being that with the Ability Scrappy, Fake Out is guaranteed to hit every Pokemon of every type, ensuring that Lopunny has easy access to its tremendous 135 speed. This easily outspeeds most of the ORAS meta (including Greninja, Mega Manectric, and other Pokemon commonly used). Not only that, upon Mega Evolving, Lopunng also gains access to a 136 physical Attack Atat. Lopunny can also switch in on a Pokemon who has just used a status move and lock that Pokemon into that move with Encore and if that's paired up with Power Up Punch, it's a deadly combo.
I hope this list helped you out somewhat with what to expect in future battles on our site