I get into this discussion with a friend of mine all the time. He likes ROTJ more, but I'm all about ESB. While the ending is open what with Han and all, it still feels like it came to a conclusion because of all the other story plots that were concluded, which were Luke's Jedi training and Darth Vader's mission about Luke (not to mention the most epic story twist of all time) and the escape of the Rebels.
I think it's these other resolved plots within the film that give it the ability to stand on its own as the middle movie. However, very few trilogies share this; mainly the Matrix and Pirates of the Carribean trilogy. Those two series' middle movies don't feel complete because they don't seem to do anything other than introduce new villians. Those two have to be viewed with their 3rd part just to make it seem like a single story, unlike ESB.
While ANH ended in a way where it could be viewed as a stand alone story (and with the two examples I used, their first movies are stand alone as well), the sequel picked up in a manner that worked. It made sense that the Empire would return to Yavin IV and drive the Rebels off of their base after the first movie, and that they would then set up another hidden base while the Empire pursued them. But with the other series, mainly the matrix, it didn't flow as well with all of that business with those other "programs" with the names I can't remember right now, and the odd loops that Neo had to go through just to get to the credits and a three month pause until the story could be completed. True, Empire ended in a way that made you want to see the next part, but the general feeling from the movie is that you watched a complete cinematic experience, and not just a half that would be completed later.
Kreshner did a great job with this movie, and I wish that he would have been able to direct the entire prequel trilogy. Now that would have been something to see!