Goodfellas for me. I've seen it so many times that you can't imagine that you'll ever want to see it again, but if it's on and you happen to watch it for a few moments, you're instantly hooked again! I think the story, the acting, the direction, the musical score - it's all perfect. I'm also a sucker for The Dambusters - have to watch it every so often!
Once upon a time in the West. The Western to end all Westerns (sadly- there have been very few since). But it's so much more than a Western - an allegory of the history and politics of America, and an elegy for the passing of the old West.