1. Star Wars (1977 version). For me this one sets a standard that the others just don't quite reach. Maybe it's notalgia, or the fact that I've seen this one more times than all the others put together, I dunno. This one is just my absolute favorite. Maybe it's the simple innocense of being able to watch it without thinking "oh, I bet they did that because it would make a good action figure/playset/t-shirt/plush toy". I can't think of a single change that was made in the 1997 version that I consider an improvement.
2. Return of the Jedi. I love the throne room scenes between Luke, Vader, and the Emporer. Particularly how the tension builds, and the music reaches a crescendo, and it's like the entire trilogy has been building to this very point since the first frame of the first film. Plus, Princess Leia's outfit in Jabba's Palace was the stuff of pleasant dreams for many years. On this one I'm torn between which version I like. I don't like the new victory celebration music at the end of the 1997 version, but I much prefer the 1997 version of Boba Fett's demise.
3. The Empire Strikes Back. The problem with this movie is that the scenes on Dagobah are boring as watching paint dry once you've seen them once or twice. I usually refer to them as the "Dragobah scenes", because they drag on forever. If there was a version with those scenes edited out or down, I'd probably like this one more, but still probably not quite as much as Jedi. The changes in the Special Edition of Empire don't really matter to me one way or the other, although I'm very much in favor of the reported change on the DVD of having the Emporer played by the same actor who played him in Jedi.
All I'll say about the newer films is that I'm not a fan of the prequels, and leave it at that.