Swann. Could've been Ajmal, but his action sets him back IMHO. Shakib would've been up there had he managed a better strike rate. He should be a bit more aggressive.
It's too early to judge Narine. Let's hope he doesn't go the Mendis way.
Dan Vettori is the best defensive spinner in world cricket. So, I would say he's the best in T20s right now.