Definitely Blizzard, this game won't grow old for quite a while.
Diablo II offers an extensive skill tree for 5 classes, which not only sets each class apart from one another, but it also allows even the members of the same class to be different, making multiplayer experience much more fun (Summoning Necro + Aura Paladin, anyone?) and deadly. Because interaction with party members is much more evident and advanced (and in some cases, necessary) than Diablo I, Experience in the multiplayer section probably won't improve much, although as Blizzard they might pull out another whammy with Diablo III.
Single-player gameplay is also exceptional, offering variety of enemies and settings which include anything from towers, occasional rare monsters, deserts, and specters in the middle of void. Needless to say, the slash-and-hack gameplay isn't limited to just dungeons unlike the old Diablo, making the experience lot more long lasting than Diablo I.
If Diablo II lacked something, it comes in the form of soundtrack. Diablo II isn't really noted for memorable soundtracks (with notable exceptions such as that for Hell), a hallmark of any good RPGs, and even if Diablo series is a horror game, there isn't any reason not to have a memorable soundtrack. However, Blizzard certainly made a classic that would not be easily erasable, since I play the classic games of Blizzard even nowadays.