Best Classic? Classic is a bit broad. . . . but here is my list.

Wolfenstein 3D ( the Grand Daddy of them all. Catacomb came out first, but was more magic based, also made by id.)

Doom (another original and pushed the envelope)

Spear of Destiny (need I say more)

Blake Stone: ALiens of Gold ( a personal fav of mine)

Those are the best CLASSIC FPS, now on to other awesome ones.

Goldeneye

Perfect Dark

Half Life

Half Life 2

Thief

Crysis

Crysis 2

Far Cry (the first one)

Doom 3

Aliens vs. Predator

The Original Halo's, CoD's and MOH's were fun, but then. . . well the hype machine has made them what they are now. I dodn't include the BF series because the online play aspect changes the whole dynaimics of a tru FPS. I feel the best FPS games made are the ones that you can play alone in a dark room with either a really good sound system or headphones.

Good Topic, can be argued forever!