well, if you want an
MMOrpg, then you could do WoW

not to sound like a little kid, but it really is a fun game. very good customer base (like 4.5 million now) and yes, it has a $15/mo fee, but its worth it. it balances storyline (quests) with grouping, and PvP, whithout cutting down much in either category
for a non-MMO, i would sudgest Elder Scrolls V: Oblivion (dont know if thats the name, i know it by Oblivion. the new morrowind

)
its also a very good game, extremely good graphics, great storyline, and some next-gen stuff too, like the NPCs have theyre own lives, if you go on a quest with NPCs, and you stop, they will go on and try to complete the quest without you. and some other stuff, but im not completely sure, the main thing i noticed when testing in on XBOX360, was the graphics. they are AMAZING. personally, i dont like morrowind, but if thats your thing, then its an amazing game
-Phate