Han Solo, Boba Fett and Vader are "badass" there's "romance" and Yoda talks about spiritual stuff.

Those are usually the answers given. And often it gets thrown in there that they remembered as kids how "shocked" they were that Vader was Luke's father and they "couldn't wait" to see Return of the Jedi in theaters, but when they finally did, they hated the ewoks and were disappointed it wasn't how they imagined. So ESB becomes their favorite.

When you look at their reasons, you can see why they didn't like Star Wars as much: no Boba Fett, the Force wasn't gotten into that deep, and Vader was little more than muscle (Vader does do a lot more fighting and killing in the original than in ESB though, when you think about it)

PS: ESB isn't my favorite of the series, I'm just telling you why I think so many other people put it as their favorite. I think generally speaking it came out at a time when Star Wars was at the height of its popularity, while RotJ was more towards its public decline... and the fact that Lucas rushed through it makes it feel a little less polished.

PPS: I don't get why ESB fans hate Lando and wish he died in ROTJ to "atone" for being forced to turn in Han & Co. (before helping save them). In ROTJ he didn't have much to do for most of the movie until the very end, but I prefer him as a "good guy" in his prime. I guess if you just want him to have more screen time, ESB is better for Lando fans.