I played around with Diablo 1 for a very short time, but didn't like it much.
Diablo 2 is a good fighting game, but has no real role-playing elements, and while it is certainly the best game of its type I have ever seen, the lack of anything except combat costs it in terms of replayability, IMO.
Dungeon Siege has probably got the best graphics engine ever invented for an RPG. Unfortunately, everything else about the game is terrible.
Divine Divinity and Lionheart are more directly comparable, as both have an excellent mixture of RPG and combat elements at the start, but tail off into hack and slash by the end. I am far more likely to replay DD than Lionheart though, so obviously I think DD is a better game, though Lionheart does have a better character build system <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
In order of my own personal preference for the four games listed, I would have to say:
Divine Divinity
Diablo 2
Dungeon Siege
Diablo 1
