Blu-Ray is the best quality you can get, but you can't get any better than the original film elements. Some of which are better than others.
But if you want to talk quality, think of this: When you watch 2001
on Blu-Ray, you can actually see that the sky in the Dawn of Man sequence is a film projected screen (because that whole part of the film was shot in a studio, indoors). That's because the quality of BR is so good. Before, you couldn't tell that the sky was projected on a screen. Now you can.