Favorite: Empire Strikes Back
It's the pitched romance, the gloriously angsty color palette, the iconic lines, the pervasive gloom of it all, and, most of all, the MUSIC. Although RotJ's music is equally emotionally important to me, ESB introduced some of the greatest themes known to man, and includes my favorite one-off piece in the entire John Williams canon, The Asteroid Field.
Least Favorite: Revenge of the Sith
The prequels are so hard to rate, since they all have a few great moments slipped into their overall ridiculous terribleness, but RotS definitely has the most laugh-out-loud I-can't-believe-they-thought-this-wouldn't-be-absurd idiotic moments. Palpatine's Jedi murder spree is awesome, but then ten minutes later Anakin is killing younglings offscreen with absolutely no ethical or emotional struggle. The Anakin-Padme sad-music-across-Coruscant scene is pretty great, but then twenty minutes later Anakin says "If you're not with me, you are my enemy." I know it's impossible to know what will work onscreen and what won't, but the sheer number of obvious flubs in this movie is just astounding.
Also - officially the best thread resurrection of all time: