As much as I'd love to say Andrei Rublev or Winter Light, the true answer is Knocked Up. It's partly because, for some reason, it's on TV in the uk around once a month. But also it's just so damn watchable. The cast are funny and attractive, and personally I think Rogan and Heigl have amazing onscreen chemistry. I also find the Mann/Rudd subplot really engaging. Apatow is way more sophisticated than the style of his comedies suggests. There are a lot of great insights into relationships and marriage in Knocked Up, as well as ageing. Some of the arguments between the couples are brilliantly written. I think the action goes a little OTT towards the end, which is a shame. One thing that frustrates me - I'm sure the first time I saw it the didn't end up together in the end. I remember being impressed by this - it was an interesting ending. Now when I watch it they do. I'm sure there's an alternate ending, but I can't find any mention of it online. I asked on this sub once but got no replies.