Oblivion and Fallout 3 are both award winning games that have great character customization, you can go on the path of good or evil (you will be treated differently by the characters in the game depending on your chosen path) and for each game the main quest can be put off and you can do the huge number of side quests or explore the huge land of Cyrodiil in Oblivion or The Capital Wasteland in Fallout 3.

In Oblivion the game does not end once the main story line is complete, and there are factions you can join (Dark Brotherhood, Thieves Guild, Mages Guild, Fighters Guild.) and there is an expansion pack in the game (The Knights of the Nine) and an expansion that comes with the GOTY edition (The Shivering Isles) I have played the game through multiple times, it never gets old.
Unlike Oblivion, once you complete the story in Fallout 3 the game ends so make sure you save many times.

Warhawk. Period. I can play that game forever.

LittleBigPlanet almost provides endless possibilities, but they’d have to be into the style and gameplay of it. Metal Gear Solid 4 is a lengthy game, has lots of extras and an online mode, but again, isn’t recommended if they’re not into those types of games. On the shooters side of things there’s Resistance 2 with 60 player online matches and 8-player co-op, and as mentioned before the Call of Duty games are constantly being played. There’s online only games like Warhawk or Socom: Confrontation, but Warhawk won’t have as many people online anymore and I’m not sure if Socom is bug-free yet. In my personal opinion you should go for LittleBigPlanet or MGS4, but it all depends on what type of game your friend likes.

def the COD games. i have 4 and 5 (four is better)