I don't think you can take half of the story and count it as one film. Star Wars is six 'episodes' (and one story) take 'em all or leave 'em all.
Lord of the Rings would be a great choice because there's tonnes of story to keep you entertained in those eleven or twelve hours. But still, I'd go with... The Dark Knight. I love that film, and it's utterly fantastic so that. In IMAX.
This may be a weird pick but O' Brother Where Art Thou?, why because it reminds me of my dad, its like his favorite movie and and I really enjoy everything about it. Also its not to long not to short and makes fun of the klan whats not to like? Second pick Indy 3 (connery, man come on)