Dazed and Confused. Not just my favourite stoner flick, but perhaps my all-time favourite film. Honourable mention for Saving Grace because of sentimental reasons.
Mine goes way back. "The Philadelphia Story", starring Cary Grant, Kat Hepburn and Jimmy Stewart. Directed by the outstanding George Cukor.
I watched "Strange Invaders" once when totally blitzed. I thought that was the best movie I ever saw in my life. Later I watched it again not stoned. I thought, 'How did I even manage to watch the whole thing?" Watched it again, stoned again, it was really good again.