Well, personally I'm split between two choices:
The Resistance series is awesome for being generic in gameplay but brilliant in action and design. They've got the most creative weapons: guns that shoot through walls and project shields, guns that shoot out tags that make the other bullets home in on one place, guns that shoot out homing buzzsaws that swarm the enemy, grenades that release gas and ignite them upon your command, and a pistol that shoots out explosives that you can detonate on command. It's just so much more fun playing the game when you are given a couple dozen unique guns rather than just 50 stat adjustments to the same 4 types of guns you've already used in every other game since Wolfenstein.
Timesplitters 2 is great because there's more to it than "get through this huge enemy-filled area alive, kill these things, get back out alive." For example: a spot with invincible golems in a jungle temple. Your weapons can't kill them, but there are pressure plates around the temple that open up trapdoors leading to spike pits, so you have to lead them over the pits and drop them in. There's plenty more to it and all, and the other thing that's cool is that there's a huge variety of levels. There's a level in prohibition-era chicago, one in Soviet Russia, one on a futuristic planet, one in the aforementioned jungle temple, and plenty more. All of the levels have a wide variety of era-appropriate weaponry, as well as their own creative backstories. All of it is tied together seamlessly to the main story, of course. All in all, it's best for its story creativity, its wide array of experiences, and its use of gameplay elements other than just direct combat.
