Any of the Lannister siblings works for me. With Tywin, Olenna, Arya, Davos and Petyr being some honorable mentions.
I'll go with Tyrion, although he has been kind of underwhelming this last season (bitching about Danny's justice, giving inefective advice, barely drinking and pissing off other people, etc)... but considering alll the stuff he has done before that, I think he's still my nomber one. Jamie and Cersei are very, very, very close though.