My Opinion as a casual player:
Kingdom of the Sky
Can't give you solid reasons except that I find myself still interested in the content of that expansion versus any of the others. I am walking through some of the grey quests just for the experience. The amount of content is huge.
2nd best: Original EQ2
Kunark is okay. As a soloing pally, the quests take forever but thats just the way I choose to play the game. I am sure the experience is different for groups and raids.
What are your thoughts?