The 'purest', most simple FPS ever made has to be DOOM.
There was next to no plot (you're a space marine, there are monsters, here are guns...figure the rest out for yourself), you had a simple objective (live long enough to get to the next level) and there was no extra bullshit getting in the way of any of the fun (no side quests, no missions objectives, no friendly NPC's force feeding you dialogue al'a HL2...just pure mindless fun).
I also think it was perfect just the way it was (simple, yet effective).
There was next to no plot (you're a space marine, there are monsters, here are guns...figure the rest out for yourself), you had a simple objective (live long enough to get to the next level) and there was no extra bullshit getting in the way of any of the fun (no side quests, no missions objectives, no friendly NPC's force feeding you dialogue al'a HL2...just pure mindless fun).
I also think it was perfect just the way it was (simple, yet effective).