Kubrick was a genius. I wish I had multiple choices here, because it's quite difficult to choose just ONE.
I love Clockwork, probably it's my favourite but Shining is very very close (Nicholson is my fave actor
) so I should vote for both these films.
I really like also Full Metal Jacket, 2001, and the two most underrated Kubrick films: EWS and Barry Lyndon.
Ok, my vote is Clockwork.