Pretty sure this thread has been done before, but alright, my top ten FPS are as follows:
10) Medal of Honour: Frontline
9) Halo 2
8) Halo 5: Guardians
7) Doom
6) Doom 3
5) GoldenEye
4) Killzone 2
3) Halo 3
2) Halo: Reach
1) Halo: Combat Evolved
Since you asked "what's the best FPS game you've played and why?", that does kind of behold me to explain why Combat Evolved is #1. Thing is, you can easily point to games that had more impact on the genre (e.g. Doom) or other games that are more mechanically sound (e.g. the later Halo games themselves). Still, Combat Evolved does get the #1 choice because it's the most...well, "fun" game on this list I've played. From the gameplay standpoint, every level is solid, the weapons are solid, the enemies are varied (Covenant, Flood, Sentinels), all of which require different tactics, and different weapons have different weaknesses/advantages over them). The vehicle combat is a blast, the health system I think is a good middle-ground (e.g. regenerating shields, but not-regenerating health), and it's the perfect mix of rewarding cover in combat, while also rewarding mobility.
On the narrative side, Combat Evolved isn't the deepest story in the world, and plenty of games in the series do have better narratives. Still, it's engaging enough to keep me going forward, and one thing Combat Evolved does better than any other Halo game is the sense of awe of being on the ring. Not that Halo invented the idea of a ringworld, and it isn't the first time you visit one either, but stepping out onto the surface for the first time, making one's way through Forerunner architecture, the music (of course, every Bungie Halo game had excellent music, not so much the 343 ones)...well, yeah. Subjective, I know, but shadup, it's my list. ^_^