I played both Diablo and Diablo 2. Diablo 1 I played single player only simply because at the time having your own internet connection was kinda special. It was fun, always hunting around for better items and seeing entire legions of enemies fall to your lighting spells, but after playing it for a few weeks the endless dungeons start to make it all very boring. Hellfire had only one fun addition to make: the cow in the field. I quote:
"moo. moooo. Look, I'm a cow okay!"
Diablo 2 was a blast to play online up to and including the 1.09 patch. Then the 1.10 patch was released and the game basically went to hell. Farmers and people using exploits completely destroyed the gameplay. The need for uber gear (which was impossible to get, except on e-bay) made the hell difficulty impossible to survive.
Now I play Guild Wars. It has all the good things of Diablo 2 and none of the bad (except for farmers screwing up the economy). The only problem with Guild Wars is that the game itself is no fun to play unless you MAKE it fun to play it, it actually requires that you take on a role in a team and play it to the fullest! Too bad about 25% of the players are just to stupid or ignorant to realise that.