I love all Stanley Kubrick's films, with the exception of The Seafarers and anything before that because I have yet to see them. I can easily say he is my favourite because no other film-maker manages to keep me fixed to the screen like he does. It has a lot to do with how they look, but also the content, themes, acting, dialogue and everything else. People say he his films are cold and sterile, 'too calculated' almost. But really he shot a shitload of film, trying all kinds of things until he got them right, so in the end it's no wonder the final product seems near perfect.
I think what makes the acting in his films so interesting is that he prepped the actors much like a stage director would prepare actors for their roles. Which means countless rehearsals before the actual 'shooting'. Another director who works similar to this (with regard to the actors) is Mike Leigh. If you haven't seen anything by him, I'd recommend 'Nuts in May', as it's my favourite :D
Here is a nice scene from it:
Nuts in May form part of the films he made for the BBC's 'Play for Today' series.