Tricky. FPS is one of my least favourite genres (at least within the genres which I actually play), but I loooooooooooooooove BioShock and BioShock 2 to the extent they must be in my top five of all time. That said, they aren't 'pure' FPS and I don't normally associate them with the term.
So technically it's BioShock 1 & 2 by a mile, but when it comes to more standard FPS gaming I'd say either Call of Duty 3 or one of the early PC ones like Ken's Labyrinth, Wolfenstein 3D or Blake Stone.
I 2nd the recommendation of "
Blake Stone: Aliens Of Gold" as well as its sequel game called
"Blake Stone Planet Strike" both of these Blake Stone games are Wolfenstein 3D clone games, but with a graphics uplift.
If anyone can buy themselves a copy of
"Blake Stone Aliens Of Gold" and
"Black Stone Planet Strike" on steam just do it, it should be cheap and both games have a TON of replayablisty.
in either of the Blake Stone games for PC, you can shoot all the enemies and all the good guys too or you can get series and collect all the hidden treasure throughou the level, you get a higher score for
1: not shooting the good scientists,
2: for shooting all the bad guys in the level
3: Collecting all the hidden treasure, there is a map and also this game lets you go to any floor in the game once it's unlocked letting you backtrack to previous levels