I'll start.
1) Farcry 3
- Had the best stealth mechanic I have ever experienced within a FPS. The multiple ways to be able to play any mission caused me to hold this in high regard. Your character's development as he slowly desends into insanity and the thrill of the kill kept me hooked on the story. Ultimately I view this as the ultimate FPS, all should work to equal or surpass this.
2) Combat Arms
- A rather simple MMOFPS that has some of the greatest guns and maps I have ever used. The map Oil Rig is one of the most enjoyable, counter-offensive, maps I have played to date. Beware though, this game is a micro-transaction whore. Protect your wallet.