I think it's because it's where we find the best versions of the characters.
Luke: Luke is not the whiny, innocent, or powerless farmboy from ANH. He is mature, confident and powerful. He is a leader or the Rebellion and the guy we know will win the day. He is exciting! He thinks he can take Vader at the end of the movie and we're inclined to agree.
By the time RotJ rolls around, Luke is humbled and serious. Knowing that <spoiler>Vader is his father </spoiler> has also cooled his sense of action and revenge. This is where the character should go, logically... but it's not the best verion of him. He's more exciting when he hasn't yet tasted defeat.
Han & Leia: This is the love story, right? Their characters are a little thin in ANH and they are really three dimensional in ESB. Again in RotJ, their characters are sort of already at the end of their arcs. Han is whipped, Leia is a slave and then a boring rebel then an crappy speeder bike pilot, an ewok captive and eventually a wounded rebel.
Vader: Similar to Luke, Vader really feels his oats in ESB. Regardless of what Vader's motivation in serving Palpatine, The Empire and The Dark Side in ANH must be, he is on the hunt for one thing in ESB and it is <spoiler> HIS SON! </spoiler>. He doesn't take crap or failure from anybody and he is on a mission. This is the best Vader. The story naturally takes him to a place after fighting Luke where he begins to change. I think the RotJ Vader is the right Vader for that movie, but he's still less interesting than he was in ESB.
So, that's why it's the best. In my humble opinion.