Mine would be Halo: Reach.
It was the first FPS I really got into, and got good enough to actually enjoy the game. Plus, there is so much that happens - you are fighting, and giant shells are flying overhead, some space ship is flying around, and if you walk over, it will start attacking with you.
In all other games I played, you could always find a 'safe zone' where some of the background things are just decoration, and can't hurt you. You could walk up and wave at them.
Plus the giant shells flying overhead were terrifying. Especially when they blow you up.







