Empire Strikes Back or Revenge of the Sith.
ESB is by far the defining title of what a "Star Wars" movie is. It's fun, action packed, with romance, all intertwined into a bunch of dudes fighting with laser swords. It's what made Star Wars so popular.
Revenge of the Sith took what made Star Wars so appealing, and then made it on crack. The action is the best in the entire franchise (Anakin and Obi-Wan fight was incredible. Dooku fight was top tier, Grievous fight was awesome, and the Yoda/Palpatine fight is one that I'll always remember). The plot was so interesting and actually made sense, compared to the previous movie in the trilogy, Attack of the Clones, which is my least favorite Star Wars movie ever. It all sets up the Original Trilogy so well, that it's impossible to not like it. I can't choose between them, though, because I love them both for so many different reasons.