#16 - TimeSplitters: Future Perfect (Kudos to Brendude for guessing this)
Incidentally the only FPS I rank higher than Team Fortress 2 is the sixth generation's finest hour. Filled with ridiculous humour; fast gunplay; hilarious characters; more time periods than you can shake a stick at and a multi-player mode that'll keep you entertained for hours, Future Perfect was the real deal.
I see a lot of people with a greater love for Timesplitters 2, and that was indeed the game which brought the series into its own. But Future Perfect improved the base game in just about every way. Gunplay was faster and switching weapons was easier; the co-op mode was better fleshed out throughout the campaign (which in itself was massively improved, having a discernable story this time). Local multi-player was as good as ever with "virus" being the undoubtable highlight but modes like monkey assistant bringing some fantastic options. Maps were as varied as ever and you could of course play as a variety of ridiculous characters.
And zombie-mansion in co-op was hiliariously awesome: