I like Empire the best.
In lieu of my disagreement with my brother about why Return of the Jedi is the best, I really don't like the Ewoks. At first, they are savages who are going to kill and roast our heroes, including R2 (would love to see how that would work out of them).
Then all of a sudden, we get this light-hearted story and now they're a bunch of lovable teddy bears? And then go on to beat an Imperial ground division? WTF.
Plus, I would like to have seen some more ship-to-ship fighting. In Revenge of the Sith we see Republic and Separatist battlecruisers duke it out with huge mounted guns. I would really have like to see just how powerful a Star Destroyer really was when it opens up and unleashes on the Calamari cruisers and Rebel fighters.
All in all, I like Return, but it just cannot match the mean, dark tone of Empire. A New Hope made the Empire look like a bunch of bumbling idiots, and the sequel totally redeemed them.
What's also really cool is that there was a rough draft script for Empire that was set to the low-budget designs of the A New Hope. Lucas wanted to do all three of the original trilogy regardless of success. However, since A New Hope was a smashing commercial success, he didn't have to cut corners and instead was able to made the big budget version of Empire we all know and love today. I would still like to see what the original low-budget draft would have been like.