As a kid, I loved, loved, loved the trash compactor scene. It was such a predicament to overcome (the monster, walls closing), and it just plain looked cool. I remember in winter I'd go in my grandmother's backyard and slosh around in a big puddle, where I'd pretend the bits of ice floating on the surface were like the garbage floating in the water.
Nowadays, I'd say my favourite scene is a toss up between Ben looking over at Luke in ANH just before Vader kills him, or (and I hate admitting this) the scenes in ROTS where Anakin and Padme stare across the city at each other with only the Brian Eno-ish music playing, or Obi-Wan saying to Padme, "Anakin is the father, isn't he?... I'm so sorry," just before he leaves to go kill Anakin.