man it's too hard to choose, I really love just about ever game I've got for the system
Here are the games I own in the order I purchased them
Perfect Dark Zero LE
Condemned
Project Gotham Racing 3
Kameo
Dead or Alive 4
Tony Hawk's American Wasteland
Amped 3
Ridge Racer 6
Call of Duty 2
Burnout Revenge
Oblivion
Out of the games I've completed here is how I'd rank them against each other
1. Condemned - AWESOME game, the story was fantastic, it was spooky as hell and I can't wait for the sequel. The game felt the perfect length too and had a great balance of achievements.
2. Project Gotham Racing 3 - Best Racing game I've ever played. Amazing graphics and it caters to my racing tastes perfectly sitting squarely in-between sim and arcade styles. I can't wait for the upcoming expansion!
3. Call of Duty 2 - I'm not an FPS fan and I'm not a WWII game fan but playing the demo on my setup really drew me in and after playing through the game on Veteran it was easily one of the most fulfilling gaming experiences I've had in a long time.
4. Dead or Alive 4 - Been a fan of the series since DOA1 hit the arcades. It was one of the reasons I bought the Dreamcast, one of the reasons I bought the Xbox 1 and one of the reasons I bought the Xbox 360. Easily the best DOA game to date, it would have been nice if they went a little further in areas but the graphics update and full roster alone was worth it to me.
5. Tony Hawks American Wasteland - Been a fan of this series since THPS1 hit the PSX. not a system seller for me (since you can get it on any platform) but I really enjoyed it, the 360 version is easily the best looking version I've ever played, it might not be the best TH game ever but I had lots of fun playing it. There are a lot of things it could have done better but my biggest complaint was thatjust... too short.
6. Amped 3 - never really played the other Amped games but this one was lots of fun, I'd almost say it tied with THAW. Again my biggest complaint was that it was too short, it took me less then 14 hours to complete the whole game.
7. Kameo - lots and lots of fun, probably one of the best looking games for the console so far. Again, too short. This game might have been higher on the list if the story was a bit more interesting or if the puzzles challenged me more. Game play and graphics were top notch though.
Out of the games I haven't yet completed, here is how I'd rank them against each other
1. Oblivion - Amazing graphics, fantastic story so far and just massive environments.
2. Burnout Revenge - tons of fun, smashing stuff never gets old, lots of challenges laid out in the game too.
3. Perfect Dark Zero - seems pretty interesting, I'm having a hard time getting into it though. I'll get around to it some time when I get bored of my other games.
4. Ridge Racer 6 - it's alright, I only played the first few levels and the gameplay was pretty repetitive. My impression of this was fairly good until I played Burnout which does everything this game does but much much more not to mention it does it better then RR6 could ever hope to do.