Fallout, Mass Effect, Assassins Creed.
Fallout because theres nothing better in the entire world than a trek through the wasteland listening to Galaxy News Radio, meeting interesting people, nicking all their stuff and leaving a live grenade in their pocket.
Mass Effect because the story interested me enough to playthrough it again four times (with my level 51 or so Bastion) and enjoy it not just the achievements, it also made me buy Dragon Age: Origins which i also thoroughly enjoy, even if its easy to not know where to go.. (Borrow a compass from Fallout? The map is so immersion breaking.)
Assassins Creed because its a game for people who want an open world and an amazing way to traverse it, Assassins Creed was the first game i bought on the xbox and it was pretty harsh on me for getting "B" and "A" mixed up (years of pokemon / old habits) especially in combat, the final boss was incredibly difficult but i felt like i'd achieved something. Assassins Creed II however was easy and brought innovation (i.e. you were no longer water soluable) in a new area in which there were loads more towns and cities to explore in the fashion of the game. And then there was Assassins Creed: Brotherhood (2 1/2 wouldnt cut it apparently) a story that would tag right on to the end of Assassins Creed II but the real reason i like it is the multiplayer, Its my favourite multiplayer game at time of writing (at war with Six Vegas 2 but Brotherhood has an excuse for insta-killing you).
Thanks for reading.