You can look at the currently playing thread, those give lots of ideas. Without knowing more about what systems you have/what games you like, it's hard to say. Here are a list of my favorite Xbox games (most are under $10 now, some under $5)
Panzer Dragoon Orta
Star Wars KOTOR
Psi-Ops
Morrowind - GOTY (only if you have the time and temperment for it)
Personally, I'm on a cheap game kick, and have been playing
Tiger Woods 2005
Harry Potter - Chamber of Secrets
Dynasty Warriors 4
Metal Arms - Glitch in the System
I'd say just read the currently playing thread, and see what you like. Xbox games are so cheap, and there are many hidden gems, that it might be time to buy a few and take a chance on some games you might not have done in the past.