the best classic fps?

Marathon.

Some good ones I've never played: Shadow Warrior, Wolfenstein 3D, Team Fortress, Star Wars Jedi Knight: Dark Forces II, Goldeneye, Tom Clancy's Rainbow Six, Thief: The Dark Project, System Shock 2, Medal of Honor, Unreal Tournament, Quake III Arena, Thief II: The Metal Age, Daikatana, Tribes 2, Red Faction, Return to Castle Wolfenstein, Tom Clancy's Ghost Recon.

Some good ones I've played: Star Wars Jedi Knight II: Jedi Outcast, Counter-Strike, Halo: Combat Evolved, No One Lives Forever 2: A Spy in H.A.R.M.'s Way,

Goldeneye, Perfect Dark, No One Lives Forever, Hexen, Zero Tolerance (for Sega Genesis, yes, I'm old), Turok series was good, way too many to list!

Half-Life of course !
Duke Nukem 3D was awesome too and the first Call of Duty was nice !

doom

Duke Nukem 3D!!

Unreal, Halo, Serious Sam, Half-Life, Daikatana, Doom 2, The House of Dead, Quake 2, Perfect Dark. I was missing Turok 2 too!

wolfenstein?

Duke Nukem 3D (sega saturn) Return to the castle of wolfenstein (pc) oh and Quake 1 and 2.

Not the best thing ever but Blood was pretty awesome ! Grabbed it on Steam recently, brings back good memories !

CoD 1, Quake III Arena, Wolfenstein: ET

Just for making some fun Doom, Quake, Duke Nukem, Wolf 3D, Serious Sam will be obvious choices.

If you want great story, atmosphere, music, voice acting and not a straight-forward gameplay then you should try both System Shocks.

If you want something in the middle then No One Lives Forever and Half-Life 1 Antology

Besides the ones already mentioned, BF1942.

Half-life, no question.

Medal of Honor 1 and 2 (Underground) for the PS1 are GODLY, they're so much fun to play, music and ambience is awesome, great enemies like medieval knights, exploding soldiers, dogs (Dancing dogs I must add) dogs with machine guns an OF COURSE dogs on tanks... It's a shame they're not on PC, I would love a Steam port.

RTCW (and then ET)

SWAT 3

Quake 3 and RTCW.

Blood, Heretic, Hexen 2, they're old but awesome

Day of defeat...

Half Life, Day of Defeat, Quake, Quake 2, Hexen 2

Counter-Strike <3

Return to Castle Wolfenstein. If you can call it a classic. it came out in 2001.

Unreal Tournament (UT99) by far. If anything older, I'd go with Quake 1 or Hexen.