There is no real way for me to narrow it down to one but the first one that pops up and is close to it is:
Peter O'toole. I love his looks, his delivery, just everything about him.
Next up would be:
Anthony Hopkins Ian Holm Max Von Sydow Robert Shaw Gregory Peck Paul Giamatti Philip Seymour Hoffman Ian McKellen James Mason Alec Guinness Peter Sellers Morgan Freeman.
I've had to edit this several times as names keep coming up.
Johnny Depp - not because he's the most convincing actor but because he bothered to. I feel like there are many more talented actors who do not push the envelope the way in which he tries. In all honesty I'd say on average he misses more than he hits, but even when missing I still feel like he's hitting because of his principles. As one documentary mentioned, his career feels like 'a body of work', and as another here has said, 'he can do no wrong.' What I find fascinating is his choices in his influences for a given character. A good example of that is combining a drunk Keith Richards with Pepe Le Peuw to create Captain Jack. In "Pirates of the Carribean", Geoffrey Rush's pirate is the more consistently convincing, but Depp's is the more unique. In his best moments he seemed to create a whole new pirate that wasn't a throwback to all the other pirates IMO. I honestly believe whatever fame and fortune he has received he earned because of his principles and vision.