OK, time for the perennial co-op thread-bump
I'm going to recommend the original Perfect Dark on XBLA for a variety of reasons:
- The controls/auto-aim make it a bit easier for people who don't have as much experience with modern FPS games.
- Lots of stuff to do: Solo mission co-op/counter-op, Co-op Challenges, and a million ways to mix it up in the combat simulator*.
- Tons of stages/weapons/items to choose from, as well as multiplayer stages + weapons from the original N64 Goldeneye.
- It's fun! Get a bunch of people on Live and play free-for-all, in teams, or band together against waves of cpu-controled enemies.
- It's cheap! Only 800 points for a game with an incredible amount of replay value.
Back in the day, I preferred Goldeneye over Perfect Dark due to frame rate issues on the latter. Now that everything runs smooth on the 360, all of the extra multiplayer options + Live play make PD really come into its own. Have fun!
*Tip: Before customizing your own matches in the Combat Simulator, play through as many Challenges as you can. Doing so will unlock a lot more options/gear for when you do your own custom matches.