2001 for me, its got so much content in it, its an amazing movie, makes you think about our reallity with all the technology and religious topics its a masterpiece.
The other on that i had a blast watching was Dr Strangelove. I Liked Clockwork orange but for me it wasnt as good as his other films
I think 2001 is probably his "best" film, overall. The first time I saw it, it blew me away.