I absolutely loved Halo 3, in spite of its brevity. I really like the 'pace' of the Halo games. Rather than an adrenaline rush from start to finish, the pace picks up and drops off throughout, keeping it interesting.
I really Call of Duty 4 even though I thought I'd hate it. Although most FPSs are quite linear, some get around this by having several wide-open areas which can be tackled by different routes & approaches. In that respect, Call of Duty didn't feel too 'limiting'. Good storyline, good variation in the gameplay and locations, better atmosphere than most action games.
Gears of War is similar, I guess. A very linear FPS, but is still a lot of fun. Looks good, the ability to interact with your environment (by ducking behind objects for cover) and over the top weapons make it fun.
(Incidentally, if you try and like Gears of War and like it, you'll probably like Army of Two as well which is pretty much the same gameplay in a current setting.)
Forza 2 I'm currently hooked on. With the driver aids turned off, some of the cars are serious pigs to drive; but it can be quite a lot of fun tuning them to make them driveable.
DiRT is another good racing game. Graphically better than Forza, with much more variation in racing types (cars, trucks, off-road). Possibly a bit more arcade-y, though not much.
Assassin's Creed - is visually stunning, has a great storyline IMO, and the interaction with the atmosphere really makes this game special. The tasks/missions are highly repetitive, but it's so much fun running around and starting a fight, it's worth playing.
Crackdown - slated by many, but I liked it. As you play and fight, your powers ramp up so by the end you're climbing the skyscrapers and leaping from roof to roof across the city. Great fun!
