D2 has far more content, better gunplay, better movement, more subclasses, more weapons, more raids, more dungeons, more everything. The only thing you could say for D1 - better atmosphere (which is clearly subjective) and all of the story bits are still in the game. That's it. In all the other aspects it's not even a question, unless your nostalgy is through the roof. And I love D1 as well, don't get me wrong, but D2 is a better FPS no doubt.