Two that come immediately to mind are-
Anthony Perkins:
Though he never really had the opportunity to do much with his career, I can't help but be enchanted by the guy every time he appears on screen. Most notably, for me, in
Catch-22 (1970). But I think Orson Welles' interesting attempt at an adaptation of
The Trial (1962) shows how his appeal can carry an entire film.
And the other one that comes immediately to mind, don't ask me why, as this seems as arbitrary to me as I'm sure it will to anyone who reads it, is Mark Lester. Not in general, he didn't really act much, but just in Carol Reeds'
Oliver! (1968).
I mean, he's absolutely mesmerizing. You can not, for one moment, take your eyes off of him.
I was at times absolutely baffled at the emotions his basic likeness brought out in me. I didn't know if I wanted to adopt this kid or marry him or what.
Of course, I agree with a few of the ones already posted as well. These are just my own two cents, I guess.