As for me, definitely Aizen and Kenpachi. Aizen's Shikai is enough killa, able to pretty much do what he wants with his opponent, and can toy around with them.

Kenpachi...oh man. He's strong enough without Bankai. The whole Bleach universe would collapse at the release of his Bankai if he got one.

So yeah, post who's Bankai you want to see, for the ones who haven't been revealed yet.

Ichigo and byakuya kuchiki .....