I love both 2001
and A Clockwork Orange
and don't really have much to add. The ambition of 2001
continues to impresses me; the attempt to convey something utterly alien, timeless and outside human experience was, I think, very well realized.
For A Clockwork Orange
, the themes are carried strongly from the novel, it's probably Kubrick's adaptation closest to the original. The ending is changed, though, and I have to cop out and say that the movie ending works great for the movie and the book ending likewise. Both wonderful achievements.