I like Pacino, but I just can't see the "best actor" comments. His acting technique is almost as comical as William Shatner's. Shatner has "pause acting." Pacino has "shout acting."
I would probably vote for a Brit for best actor ... Anthony Hopkins, maybe. I'd even prefer an Al Finney. These guys can do anything without imposing their characterization onto character. Pacino is always Pacino, no matter who he's playing.
In terms of Americans, I'd probably go with Kevin Spacey as the guy who covers a lot of territory with credibility.