10 is a lot.... I'll do 5.

1. Counter-Strike
2. Call of Duty: United Offensive
3. Call of Duty
4. Counter-Strike: Source
5. Doom

top 5 takes less time, and in no particular order

DOOM 1&2
Goldeneye
Half Life 2
Quake 2
Perfect Dark

Honorable mention: Call of Duty games. Counterstrike. Team Fortress 2. Duke Nukem. Return to Castle Wolfenstien. L4D

I would not include most FPS released recently since they all seem to be "generic shooters". IE: rehashed shooting games with no originality or variety. Games that only are fun during multiplayer. I won't name any to avoid aggravating fanboys.

Bioshock would have made the list, but the vita-chambers removed all sense of danger from the game. Who cares if you die? You just get resurrected in a nearby chamber.

Also, when asking which FPS is the best, you have to differentiate between most original (usually older games) and best designed (usually newer games).

@ fuegonian
Resident Evil 4 is not an FPS

I'll be back.

Just Killzone 2 for me.

Going with anything that resembles FPS and excluding MP only titles (because I always want SP/MP with the same title) I'd say:

1) Deus Ex
2) Half Life series with Half Life 2 the stand out for me
3) System Shock series with SS2 the stand out for me
4) Halo CE (on PC)
5) Doom series (Doom2 baby)
6) Unreal
7) Stalker SOC
8) CoD series (4 probably best for me)
9) Quake series
10) Stalker SOC

I have poured insane hours into the following MP, so they get an hounerable mention:

Doom
Quake
Day of Defeat
Counter Strike
Unreal Tournament
Unreal XMP
Enemy Territory

- Goldeneye
- Half-Life 2
- Shogo
- Medal of Honor: Allied Assault
- Team Fortress 2
- Tactical Ops: Assault on Terror (I just HAVE to include it)
- System Shock 2
- Battlefield 2142 (yes, 2142)
- Day of Defeat: Source
- STALKER

I guess that's it. I have UT2004 but I don't include it because I didn't exactly play it, I played mostly TO:C and Frag.Ops

Doom 2
HL1/Counterstrike/Team Fortress Classic
Deus Ex
System Shock 2
Quake 2
Unreal Tournament
Goldeneye 64
Duke Nukem 3D
Rainbow 6: Rogue Spear
Operation Flashpoint

That's probably my top 10. In no particular order.

1) Counter-Strike
2) Team Fortress 2
3) Half-Life 2
4) Killzone 2
5) Resistance: FoM
6) Goldeneye
7) Call of Duty 4
8) Bioshock
9) Resistance 2
10) Quake

IN order is going to be hard...right now I'd say:

1) Half-Life
2) System Shock 2
3) Duke Nukem 3D
4) Halo CE
5) Clive Barker's: Undying
6) Halo 2
7) Goldeneye
8) Half-Life 2
9) Return to Castle Wolfenstien
10)Dues Ex

Outside of the top two, I could probably re-order that list 100 different times though...

Starting at #1:

Half-Life 2
Deus Ex
Metroid Prime
Duke Nukem 3D
System Shock 2
Half-Life
Battlefield 1942
Unreal Tournament
TimeSplitters: Future Perfect
No One Lives Forever 2: A Spy in Harms Way