I pretty much agree with HandEFood except between the Diablos...
Divinity is unmatched for the amount of gameplay you can get out of it. Every quest I do seems to bring me two more to work on after that...I love it. And even though I've played through it once before, I'm still getting quests I never encountered before.
Diablo 2 I've beaten several times. However, at least half of these were because I played through it multiplayer with a friend. That game you can beat in a day though, because the quests are so straightforward and unchanging that it just gets ridiculous to repeat the game because there's no variation to it.
Diablo 1 (I thought) was a freakin awesome game and I beat it over and over...but in retrospect it wasn't as good as I thought it was. It really was just a new sort of game for me.
Dungeon Siege had my hopes up with its looks; however, as everyone's been saying, the storyline blew...it's really too bad. Same for Neverwinter Nights in my opinion.
When all is said and done, I believe that we're going to have to revise our lists to throw <img src="/ubbthreads/images/graemlins/riftrunner.gif" alt="" /> at the top <img src="/ubbthreads/images/graemlins/winkwink.gif" alt="" />