As much as I love PSH and admired him in Capote, I think he's had much better roles, and I think he does greater things than impersonation. I voted for Heath Ledger.
I had really hard time deciding between the two--I don't feel quite so strongly that Hoffman has had much
better roles as you, JB--but I also ended up voting for Ledger. Of the two characters/performances, I decided it was the tougher job and more of an achievement. Still, given the roots of this board, it's not surprising that Hoffman was a shoo-in.