Without a doubt I'd say Arnold Swartzerneggar - with Sly Stallone a close second.
I just recently shot a movie with both of them that will be released next summer and I got a chance to watch both of these guys in action.
I don't think Arnold can actually run anymore - he skulks around as if every step is a chore for him to make. He did NO stunts - no running scenes. He's a nice guy and very "extrovert" and smiles a lot and he's nice - but his body is old beyond it's years.
Sly - walks as if every vertebra in his spine has been fused - like he's got a stick running from his buttocks to his neck. When he sits down - he sits that way too. He CAN move though, relatively quickly. I saw him demonstrate a "clean and jerk" technique for Arnold between shoots.
It's sad - cuz both these guys were my heroes. Hard for me to believe that, at age 50 - I'm a youngster compared to them.