My thoughts on the current metagame...
Kyurem: dear god, is this thing annoying. Earth Power and Roost were the best moves Kyurem could possibly ask for, which fix most of its old problems. Its above average bulk and Pressure ability are now put to good use, being able to stall moves like Close Combat and Stone Edge very easily. Its Ice typing is suddenly a blessing in this metagame full of pokemon weak to ice and Mamoswine ready to Ice Shard them to oblivion.
What makes it even scarier though is its sheer versatility despite its relatively small movepool - you could expect a SubRoost set, only to be vaporized by a Specs STAB Draco Meteor coming from 130 Sp.Atk.
Planning to wall it with your pink blob of choice? Say hi to the dreaded Sub+Hone Claws set, which sets up on those.
Last but not least, thinking to send Scizor in and bullet punch the icy dragon? Hidden Power Fire is still there to roast it.
Kyurem does have one glaring weakness though: it's very vulnerable to status. Even burn will cripple it badly, since it's also weak to SR and Roost can only partially fix that problem.
Tornadus-T: probably the best stat distribution in the entire OU tier. Nothing goes to waste. It has the bulk to survive most neutral attacks, it has a very trollish speed stat and it has good enough mixed offenses to give it a degree of unpredictability.
Even though Tornadus-T loves rain, it can function in sun well thanks to pseudo-STAB Heat Wave.
Regenerator is also a fantastic ability to have for obvious reasons. The best of the Therian formes in my opinion.
Keldeo: I'm somewhat unimpressed by this pony to be honest. It's a great late-game sweeper, but easily revenge-killed by Tornadus-T, Psyshock Lati@s and Starmie. Amoongus is a relatively good check as well.
Sunny Day Venusaur also screws him over.
Thundurus-T: the Agility set is checked by Ice Shard Mamoswine, while the Nasty Plot set is checked by Lati@s. That's about it. If you don't pack these pokemon, Thundurus-T is a real bitch to face, but its frailty is a major weakness. Sun also makes it a lot less threatening, unlike Tornadus-T who can still hit hard with Heat Wave.
Breloom: honestly, way overhyped. With Tornadus, Salamence and Mamoswine everywhere Breloom is not that dangerous. The loss of Poison Heal also makes it very susceptible to Will-o-Wisp and Scald, making it even harder to switch in.