Well, i voted for Christopher Reeve.

He was the first Superman I ever saw and for me, he'll always be the definitive one. He put on loads of muscle for the part (contrary to popular opinion, the body was all him, no padding) and did a damn fine job of becoming The Man of Steel.

I really like Tom Welling as Clark Kent in Smallville though. Because of new technologies, we see much better action than we did in the Superman films. Still, he's not as definitively 'Superman' as Reeve, mainly because he's not actually Superman, and that holds him back.