Such a hard decision. I'll start of with my fav games I got for 20 or under but my favorite game is Rockstar's Table Tennis
Monday Night Combat($7.50 Sale): I had fun with it early on but now that I have started mastering the little nuances of the game's mechanics it's become so addictive.
Super Meat Boy($10 Launch Price): Great stress reliever and serves as a good diversion when I have little time to play.
Castle Crashers($15): Fun, Humorous, tons of Content, terrific ArtStyle, and catchy music.
Rez HD($10): Demo ensnared my senses so I hit the purchase button
Resident Evil 5($20 Used): Loved playing the co-op mode with my sibling.
Halo Wars($10 New): I picked up a new Halo Wars in the past summer for $10 and I've sunk over 100 hours into it since then.
If I had to choose my number one pick it would be
Rockstar's Table Tennis. This and Halo 3 2006 trailer were the reasons I actually got a 360. I hate sports games in general but playing RTT and experiencing a 100+ rally is one of the greatest pleasures gaming can give you.