Blood 2: The Chosen

This game was very politically incorrect and way ahead of its time. Funny, gory, action packed, sarcastic one of a kind fps.

No developer would dare to make a game like this today due to its politically incorrect overtones. And that's what makes this game so unique.

I'm voting for Doom despite this. Even without all the source ports and bazillion mods, vanilla doom is a masterpiece of level design, especially E1. I played the hell out of it when it came out and I still play it. I'm old school and play pretty conservative with PrBoom+. I've played Brutal Doom and frankly it ruins the game beyond repair. If you must have gore, try Beautiful Doom, which doesn't change the tactics of the game. And frankly, it's the tactics that make the game stand up to this day. For a rudimentary FPS by today's standards, the new strategies that speedrunners keep finding in this 20 year old game is amazing.

And about those fan based mods. What other game community comes close to Doom WAD community? No, really, is there another community as dedicated to cranking out as much custom content as Doom? Because I'll play that too.

Commandos 2. The best one of the series, I still play it from time to time.

The Neverhood (I really hope it come to GOG soon)

And gladly the creators are making a spiritual successor: Armikrog

Links series (golf).