A few nominations (most of this has probably already been said, I'm not reading through this entire thread to see what has):
Charizard-X S ---> S: It's the best Dragon Dancer in the tier post-Mence, and it's Special Defense-WoW set is a pain to deal with. A big advantage it has over Mega Altaria (biggest Dragon Dance competition imo) is that it doesn't get revenge killed by Landorus-T. It's offensive DD sets also outspeed every new mega, and Jolly Aero/Timid Zam. Another advantage to using Char-X is that it's also an effective way of getting past Mega Sableye stall teams, Char-X actually uses MegaEye as set-up bait.
Mega Gyarados A ---> A+: Mega Bagworm is great with Crunch. Water/Dark is a great STAB combo, and it's got enough speed and bulk to create a lot of set-up opportunities. It can also run Jolly and outspeed every new Mega after a DD. +Speed base 150's can revenge kill, but anything below that can't. +1 Jolly Gyara can also OHKO Sceptile and Beedrill, and stands a good shot at OHKOing Lopunny. Dark STAB made him a lot better, and it's stallbreaking ability also received a boost because Crunch is a lot more spammable than Earthquake against Water resists (basically Ferrothorn, but there are a few others). It also can set-up on Sableye and Mega Slowbro regardless if it carries Sub or Taunt, which is something that can't be understated.
Mega Aerodactyl: A- ---> A: Great revenge killer. It's got a ton of options movewise, the speed to outrun everything, and decent defensive typing on top of that. It's also got a really customizable movepool and three great non-attacking moves in Hone Claws, Roost and Taunt. Running Jolly is the better option over Adamant btw. You want to straight outspeed Sceptile, Lopunny and Beedrill even if Aero has the bulk to take a hit. The stallbreaker Aero set is still very good as well, if you adjust the speed for the new Megas running around.
Chesnaught B ---> B+: Drain Punch is a great upgrade over Hammer Arm, and Chesnaught checks a ton of important threats. Lando-T, Mega Gyarados, Excadrill and Mega Lopunny are the best examples of this, but there's a lot of them out there. If also beats Ferrothorn better than just about anything out there mostly because it can't be touched by Leech Seed or Gyro Ball.
Mega Sableye A- ---> A+: I could actually see this in S, but A+ is a very good start. Everyone else has talked about how it's the face of ORAS Stall, but it can also be a great asset to Balance teams. Those teams love having the extra hazard control, and Sableye also provides a great win-con, especially against Stall teams.
Mega Beedrill B ---> B+: Very... decent. It does it's jobs well, but also has a ton of short comings. The STABs are fairly easy to wall, and it almost always has to carry Protect. It has a tough decision to make between running Knock Off and Drill run in the last moveslot, and depending on what it runs it gains a decent number of new checks. It's also hard checked by Skarm and Gliscor, and Landorus-T being everywhere really doesn't help either. It's also so damn frail that you need to carry some bulk to use it effectively. All that being said, it's still a great U-Turner and and has a great speed tier, and it punishes teams that don't have the proper resists.
Mega Swampert B+ ---> A-: Gary summed it up best, but I think it's a level up from the other Swift Swimmers because it's also a functional Tank outside of the Rain. Sure it prefers the Rain, but it's not auto-deadweight if Politoad goes down.
Raikou B+ ---> A-: Got a lot better in ORAS. Specs and Calm Mind sets are great for breaking Mega Slowbro, and Specs Volt Switch is a hell of a momentum grabber. It's also a decent answer to GunkNinja. It seems like it's going to be joining OU soon, not that a tier change would impact it's ranking here at all.
Also, 500 posts! I really need to fill my free time some other way.