Halo 3 ODST, which is Amazing. Gameplay requires a lot more thinking, as you are far more vulnerable, it's not just rush in and blast everything that moves at 600 Rounds-Per-Minute, you will spend a lot more ammo, and will have to stick to cover a lot more, the game, even on normal, can be a bitch. And DO NOT underestimate the new Engineer enemy.
To add to that, the music is amazing, and the characters are far more interesting then those of the previous Halo games. Firefight is awesome, like somebody took Gears 2's Horde mode, threw an extra set of balls on, and made it eat 10 Bhut Jolokias Peppers.
The three new maps are sweet as well, and you can finally get Recon, which is cool.