I personally like Black Ops, Homefront, Halo 3, and Battlefield 1943 (the XBOX app game thingy). I have no experience with Halo Reach or Battlefield: Bad Company (1 or 2). I have little to no experience with MW2 (on Live) but I know that tons of people play it.
GTA was one of the most technically amazing games ever when it came out. But now it's just bleh. Rockstar needs to innovate than just update same old formulae with better graphics.