Quick paragraphs for the two I voted for so far:
Diablo II
Not considered a classic by many. To those people, I would say "Shut up and go away."
Diablo II was never about an enthralling storyline, or imaginative, creative quests. It wasnt about your character having a unique and defining personality, or the quirky NPCs who accompany you (although Deckard Cain had his moments...).
What Diablo II (and basically the entire series) is about, is loot, and character development. It was addictive to the same level as gambling or smoking. Those repetitive boss-runs, gearing up the perfect magicfind character to optimise your success chances.
And then that moment when that Rare weapon drops... completely unidentified. Limitless potential. Could this be the weapon which has the stats rolled *just* right and instantly catapults you up the trade ladder? Or is it just another "of the Eagle" crap? That moment, that suspense is something I have not yet seen replicated by any game.
Diablo II was a game I invested hundreds, if not thousands of hours in, and yet never even felt like I was even in the top 50% of players in terms of "wealth", and yet it still didnt feel like time wasted. Every Baalrun, every Cow Level, just gradually edging 1 step closer to creating the perfect character.
Dragon Age: Origins
I actually just had a re-runthrough of Dragon Age a couple of weeks ago, and it reminded me of some of the reasons why it is such a fantastic modern RPG. Not an RPG like Diablo. A "proper" RPG - quests, characters, romance, and a very, very epic storyline, which always helps.
Dragon Age I would say doesnt do anything enormously revolutionary. It is in most areas, a pretty standard RPG, however it is just the quality of it - it is polished, sharp and gets most of the fundamentals absolutely bang on. Lots of replay value with different classes and storyline choices, lots of amusing conversations and quests. Dragon Age has some of the best NPC characters I have ever come across - from the sheer dryness and wit of Morrigan, to the reluctant hero in Alistair, to the simple pleasures of Dog (that didnt quite sound right...), and the storyline is incredible.
Basically a modern day equivalent of the Baldurs Gates and Neverwinter Nights of old, and that takes some doing.
Dragon Age II had very big shoes to follow and went down a slightly different route, which didnt sit right with many. I still enjoyed it as a good game in its own right though, and with Dragon Age III on the horizon to conclude the saga, these are well worth a playthrough, especially since you can import your saves as you go through the games.