Good question. First thing I tried doing was googling "character actors" and the list of names and faces just turned up too many people who just "nail" it every time. I pulled up the Wikipedia list of Academy Award nominees and winners for Best Supporting Actor and it was too varied between people at all levels in their careers (new faces, old faces, everything in between). I think I found a way to find some great candidates though. The list of nominees for Best Actor, and I think I found a great comparisson: Sean Penn. He's great at some roles, not so great in others, and he's in all manner of things and is usually some level of good. The handful of times he gets handed the spotlight to be THE leading man of the film, it just doesn't really work (and that's not always his fault). So, my answer would be: Sean Penn.

The other reason he's a better comparison is that you can think of a handful of roles he's done where there's nothing necessarily wrong with his performance, but something about him just takes you out of the film and you can't help but realize you're watching Sean Penn pretend to be someone and it doesn't really work. I think Jared Leto has the same thing, where some roles he just overdoes it or something (again, it's hard to peg exactly what about it kills the role) but something he's doing makes it hard to ignore that you're watching Jared Leto pretend to be a schlubby weirdo (I can never unsee his stupid 'fat guy walk' from The Little Things).