Elder Scrolls 3: Morrowind
Yeah, #4 (Oblivion) has been out for a while, but after a few years of playing #3, I still have tons left to do.
Might get around to Oblivion by the time I'm 30.
Truely one of the better RPG's I've played, because you can actually role play. In a lot of RPG's, especially console style ones, you follow a pre-destined path, learn pre-chosen skills once reaching a certain level etc. In Morrowind you have much more freedom than that. If you want to role-play as a Thief, the game lets you break into houses, steal from shop-owners, pick pockets and try to escape from the guards when they're called on you after being caught. If you want to play as a wizard, you can enchant armor to make your physically weak body more defended, shoot fireballs, summon creatures to fight for you, mix wild items like flowers and roots to make potions and create your own spells.
The options can be very overwhelming. I'd look into it, but only if you have lots of time to spare. I don't play it very often these past few months, but I still enjoy sitting down in the evening to a night of exploring dungeons and ancient ruins.