If you're looking to get a lot of game for your buck:
-The Orange Box. Has essentially 5 games in one. I reupped on mine at a local Gamestop for $26
-Fallout 3. You can easily put in 100+ hours of game play and not even know think about it.
-Skyrim. Another game you can easily put in 100+ hours of gameplay without even the main questline.
-Crackdown. The first is really fun, even though the story is nonexistent. It's just a fun game to play. And as stated, it can be copped for like $10 now.
Good games to cop in general:
-Saint's Row 3. VERY fun. Story is obnoxious/stupid but it's funny sometimes.
-Call of Duty 4. These games haven't been the same since.
-Resident Evil 4. It was on PS2 back in the day, but I'm pretty sure you can find it on the marketplace, if they haven't taken it down.
-Bioshock. The first Bioshock is one of the greatest games I've played. EVER. A little disappointed personally with the lack of character development from the first main character.
-Assassin's Creed series. Gameplay in the first game gets VERY repetitive. But the series overall is really good. The story is great as well.
-Mass Effect series. I've only played the second, but it was really good. I plan on picking up the first and third eventually and just replaying them all back to back.
I can't speak for any newer games because I haven't got around to copping them yet. But I expect Max Payne 3 to be