Stars Wars can count, but it has to be 1-3, 4-6 or 6-9. Hell, even Indiana Jones (you can forget about IV, which wasn't even that bad, imho).
I'm tempted to go with Back to the Future, as those movies are just so much fun.
But for me, there is no real other choice than The Godfather trilogy. I and II are two of the greatest movies ever made and III is excellent too, if not quite as great.
Here is a good list.