I'll also say that ROTJ had the coolest looking lightsabers (they sounded big, powerful and heavy, and had brilliant glows, despite Lowry Digital screwing them up in the 2004/2005 DVD's, in the original versions they look fantastic and looked great in theaters in 1997 too) and the best duel (apart from Episode I that is) and the best special effects (up to that time).
It also has the greatest space battle of the series, period (the Episode III battle is too chaotic, we don't spend much time on it, and the heroes really aren't participating in the battle, just flying through it to get to Grievous' ship).
Yes, the Emperor was an over-the-top villian (much more than the subtle mysterious figure of ESB, or even the scheming politician of the prequels), but he stole the show! (and had some of the best lines)
Yes, ROTJ's weakness is that it doesn't develop the main characters a huge amount (it presumes you've seen the previous two movies, or at least ESB), but it's still my favorite. Best action, and doesn't go overboard on the cute/slapstick/jokiness as Episode I would (though the Special Editions lay this on thicker and start to plunge into that territory... remember they also added some of that hokiness to ANH).
As for the TPM duel, I consider it "the best" from an action standpoint. The camera pulls back and lets us SEE so much of the fight and it's so fast and awesome that I can't help but like it. Yes, there are interruptions which are annoying as they are in the other films but still, what we see is just flat out amazing (the first battle with Qui Gon in the desert is much more traditional, I'm talking about the battle in the factory/foundry/whatever that is between the three and then when they split off and fight Maul one on one).
To everyone complaining about the second death star.. I feel your pain. But be glad you only got one (in an early script draft there were THREE new death stars!).
And yes, I agree that Lucas ought to have had Dave Prowse in makeup under the mask originally. But oh well. We got Shaw instead (a veteran actor that most kids today aren't going to be familiar with), but we ACCEPTED HIM for 21 years as the "true face" of Darth Vader/Anakin. Then suddenly Lucas wants us to believe that it's really Hayden! I much prefer the kindly old man, a "healed" version of Shaw under the mask, than that long haired hippie whiner we saw in Episode III. I don't like the way he's looking at his daughter there either...! Luke goes back to the fire thinking "who the heck was that weirdo with Yoda and Ben... he's younger than me!"