Okay, so I haven't really seen these movies in a while (I know, I know, I'm ashamed too), so I can't really give an accurate rank-list right now. My dad, sister, and I are planning sometime soon to start watching the OT so we can see it before Episode VII comes out. We would've done the whole saga, but I had the idea recently and my dad doesn't want to cram all 6 movies in a few months since he gets "movie-series tired" very easily (It's actually kinda ridiculous how easily tired he gets of watching a particular movie series... and also of repeat viewings. With some movies, he has to wait years before he'll want to watch them again). If I had to give a list solely based on memory (which will probably change both when I watch the OT again and once TFA comes out), then I'd say I like them in this order:
Empire Strikes Back
A New Hope
Return of the Jedi
Revenge of the Sith
*sigh* Tie between Attack of the Clones & The Phantom Menace... I haven't seen these movies in a while, so I can't really decide the better/worse of the two, though I do have my reasons for both of them being near the bottom
The Clone Wars movie (would've been better as a opening multi-part arc in the show, though at least it got people's attention in the first place and, obviously, the show got better)
Again, I think I would need to see all the movies again in order to make a legitimate ranking and have clear answers to why I rank them that way.