In my opinion all three are basically equally great. A comparison between them seems ridiculous since each has different role in terms of trilogy. Obviously Empire has the most characterization but that is purely because of the storyline. The main purpose of the middle chapter storyline is to develop the characters introduced in the first and to complicate things which offers drama.

I believe Lucas can be blamed for Han and Leia characters in Jedi. I think he should NOT have concluded the romance subplot in Empire which greatly depleted their potential for Jedi. I think Han should be frozen without the love proclamation (I know fans are very fond of Ford's "I know" but still). That would leave some tension for Jedi.