Out of all the new toys we got so far:
Landorus-T: It's actually pretty decent, but far from broken / suspect worthy. It's incredibly annoying with Intimidate and U-turn, but it doesn't have recovery. If it did, I might have just pulled my hair out.
Thundurus-T: I've been using Sub Nasty Plot and it gives any balanced / Stall team without a gastrodon trouble. Even Ferrothorn takes upwards of 70% from a boosted Thunder and while it has some difficulty setting up, it can prey on Forretress, Ferrothorn, most water types will switch out, etc. Even against offensive teams, Life Orb Thunder is OHKOing a lot on your average offensive pokemon. 101 base speed is good but not great, so it can be revenged quite easily. It still has some universal counters regardless of the set, like Gastrodon and Quagsire and some other Water/Grounds.
Tornadus-T: difficult to say here. It has plenty of checks - Jirachi, Bronzong, Metagross, Blissey, Chansey, Tyranitar, Defensive Rotom-W, Jolteon, and such. When it comes to practice though, it can still be difficult to deal with because U-turn will wear all of them down eventually. The worst thing is probably the confusion too, as you can actually lose one on one with Jirachi if you have bad luck with confusion. Blissey can switch in on Hurricane, and heal through the Superpower to keep it in check, but obviously switching in on superpower is not good. It doesn't kill it's self from recoil either, so it can bounce around a lot. Although if you have Stealth Rock down and it attacks once, you're still getting damage on it in the long run (2%, but it's better than nothing. 12% if it attacks twice) As long as you have one or two of these checks though, you really shouldn't have any problems with it. It's also pretty reliant on rain, as it's risky to throw out Hurricane with only 70% accuracy. Overall, I don't think it's broken personally, but it may be worth a test.
Genesect: Also a tough one. It's coverage is awesome, there are very few common scarfers that outspeed it ( i.e. Rotom, Politoed, Tyranitar, Hydreigon, Haxorus all fail to outspeed) However, it's defenses are meager with a naive nature and minimal investment which means it will die to a lot of common special attacks which limits it's switch in opportunities. Still it's very versatile and a great revenge killer, especially in Sun. I think this guy may be worth a test.
Meloetta: Really unique and versatile, Calm Mind + 3 attacks is actually awesome, as it really has no troubles with Scizor / Tyranitar like reuniclus, and it can actually switch in on something, unlike Alakazam. Relic Songs sets are cool, as it has good wall breaking potentail and really good speed for a late game clean up. It's not common though, and has some roadblocks for each of it's sets. Not worth a test, but I encourage people to use it since it's actually really unappreciated.
Keldeo: Again, really good, but not gamebreaking. Specs in Rain hurts anything not named Latias, Celebi, or Amoongus and even all three of them are taking a big beating. Scarf hits fast, offensive teams hard, while Calm Mind can wall break pretty well. The problem is that Tornadus-T, and the aforementioned pokemon do pretty well in checking / countering it so it's not too out of hand. Not really worth the test in my opinion.