Episode IV out of the old movies, Episode 2 out of the new movies. Episode IV would be my favorite overall. I think the only reason I like Episode II is because my church's pastor looks like Count Dooku.
Episode V was easily the best of them, story and characterization wise, but my favorite moments in the films were the attacks on both Death Stars.
My goodness, why did the stop using models, it looks a million times better. The battle over Coruscant in Episode III was just an abysmal cluster of particle effects. I loved the sort of feel in the OT that there was a military conflict, in the other three it was completely focused on the individual heroes.
Episode V; the prequels were ruined by atrocious acting (Hayden Christensen, principally) and, to a lesser extent, by subpar writing. Rarely have I seen such awkward and stilted romance dialogue as that between Anakin and Padme.