Was that not third person?
Soldier of Fortune
Half Life.
Yes, the game has aged graphically but the point being, for its time, no other game since has offered the same innovations involving gameplay mechanics and story telling and pacing remotely comparable with the same impact Half Life had in its time.
No other FPS has given me the same feeling when firing it up, from the opening scenes, you are hooked and in the palm of their (Valves) hands.
I thought the original Operation Flashpoint was amazing - so unforgiving, one or two hits and you're dead - one savepoint per level from memory which meant you had to risk going further and further before saving. Also the variability in completing your mission - remember one mission where you have to escape from a forest - spent ages trying to dodge enemies through trees - then tried something different where I sprinted out of the forest to a road, saw a jeep coming towards me, sniped the driver and got in the jeep and drove away to safety!
It's tough choice between MOHAA and Perfect Dark but I think I have to go with MOHAA.