Oldman and Hopkins are definitely up there.
I've seen some older movies with G. Oldman here lately. Can't believe what a great range he has. Freaking awesome as a good guy or the villain.
Honestly, I don't think there's one greatest actor. If you look some were the best of their day because they truly lived in the character and conveyed the ideas that meshed with the audience of the day. Some of the acting from older films is "over the top" at times in comparison to some of what you might see now, but it is amazing none-the-less.
Take someone like Samuel L. that people downplay because he is in so many movies; find a film where his presence doesn't lend weight to every scene that he's in. Not saying he's the best ever, but he gives you your money's worth and then some.
There is no best, but I'm thankful there are so many greats.