For those who enjoy FPS titles, which do you think was the genre's greatest era, and why? You can pick a generation if you want, or just a series of years of your choosing.

Late 90s/early 2000s was a great time.

Quake
Unreal Tournament
System Shock
Half-Life
Deus Ex
Medal of Honor
Halo
Goldeneye
Turok

Just to name several notable franchises. Either multiplayer, single player in story based or immersive sim, or a combination of the first two, there was a lot of to love at the time.

Good call, I'd also throw in Perfect Dark as one of that era's best, terrific game.

This is the only correct answer to this post. 

Quake 3 arena,unreal tournament.
Nuff said

I'd say from Quake 2 until Half-Life 2. From there it kinda went downhill. There were some notable exceptions later on, but at least in my opinion, the golden era ended.

In 2007 we got Halo 3, COD 4 Modern Warfare and Bioshock. I wasn't big on Halo 3, but many people were, COD 4 is my favorite COD and Bioshock is one of my very favorite games of all time. Gonna be hard to top that year for me regarding fps'.

Another good answer; that year saw the release of Crysis and The Darkness as well, both of which I thought were damn good.

I'm partial to the early-mid 90s era. Doom, Doom II, Wolfenstein, Powerslave, Quake, Hexen, Heretic, Dark Forces, Duke Nukem 3D etc.

1997 to 2008ish

Don't think i have really played any FPS that was realeased after that as they all became CoD.