Well since some time passed since Diablo 3 release... we all know what happened. Let's hear the community voice.
My vote definitely goes to original Diablo 2 - The expansion introduced nothing more than 800x600 resolution, same applies for Diablo 3 at some degree (no more ladder, easy level up, boring "end game", easy hardcore, boring skill system and many more flaws).
why? diablo III is still a big beta, with chances to improve considerably in future patches, not to mention expansions...
so far, LOD was best game altogether. but i'm nor going to vote yet
instead, i'll vote for best storyline:
1) diablo 1 (dark fantasy at it's best)
2) diablo 2 (a close call, only tad worse)
3) diablo 3 (hardly a dark fantasy at all, lots of lore faults and deus et machinas (diablo appearance - why? wasn't he massing army in hell when his soulstone was destroyed? or it doesn't matter? or he was even then planned to have soulstone crushed, and then, when 6th and 7th lesser evil - a fit opponents, considering other villains - magda/adria - decide to invade), but also some interesting future storylines - Shen/Zolltun/Baal/vizsjai (whatever) line, and boring scoundrel/lead/adria line). we shall wait an see...
4) LOD (here we go to full deus ex machina, a suddenly feminine Baal seeks some stone who is guarded by some barbarians and crucial for some reason)
5) hellfire (fighting diablo's servant - well, more pitiful even than 6th and 7th greatest evil)
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Even though Diablo 3 has pretty good graphics, I would still go for diablo 2 LOD. the gameplay feel and storyline overall is better. Also, Diablo 3 has many cliches, has too easy of a level up system, the gameplay is too easy.