1. MGS4 (It's MGS. I could play it forever)
2. Little Big Planet (all the time in the world to make my own levels!)
3. Killzone 2 (the best FPS with bots I could think of)
4. The Orange Box (4 games in one seeing as how the fifth is online. But 4 of the most top notch games ever made? OK!)
5. Oblivion GOTY edition (I like Fallout 3 more, but I felt like there is more to do in Oblivion and it has more room for multiple playthroughs due to classes like mage, knight, theif etc. which all play differently.)