Underrated:
Silent Storm. Everything in this game could be destroyed, and it had persistent damage to EVERYTHING. Bullet holes, bodies, scorch marks, everything all stuck around. I haven't seen any other game do this, even Max Payne 2 and Fallout 3 cleaned up after themselves. It's tactical combat is also very deep and wicked fun. The writing is shit, but everything else is cool.
Psi-Ops: The Mindgate Conspiracy. This game is fucking awesome. If you still own a PS2 or Xbox, you should own this game. It's so fucking fun. Sneaking up behind dudes and popping their brain with your mind, taking control of a sniper and making him shoot his own allies, tossing shit around with telekinesis...it's AWESOME. The narrative is also awesomely STUPID, but no perfect game is perfect. And the last level is a *****.
Halo series (excluding ODST). Yes, I'm saying underrated. Halo is just plain fun. There's nothing wrong with it, unless you're talking about the narrative, and I'm not arguing about that. It's also a great party game. I haven't seen another FPS that's as party friendly as any of the Halo's. All you need for a 4v4 match is two copies of Halo and an Xbox. The same can not be said for CoD, or Rainbow Six.
Overrated: GoW. Fuck those games. I honestly prefer the combat of either Mass Effect (and I'm not unaware of the flaws in those games) over GoW. Call me strange, but it's true. And as you may have noticed, a shitty story doesn't prevent me from adoring a game, but something about Gears really, really, REALLY puts me off.