Half-Life might be too plebian of a choice, but at this point I have completed three separate versions of this game by now, so its easy to say its my favorite of all time.
I think Killer7 also comes under first person shooter category, though it is a hybrid game of many genres, I would still like to put it here.
Monolith Productions was one of my favorite game studio out there, and three of their games -- No One Lives Forever (which I get is an immersive sim/stealth mixed with fps, but would put anyway), Shogo Mobile Armor Division, and F.E.A.R. are some of my favorite games out there.
I wanted to put Portal in here, but I guess they come under FPP (First Person Puzzle) games instead. Still solid games.
For a more proper FPS games, despite me not liking the series overall, I do think Call Of Duty series has a few gems and Call Of Duty 2 in particular is really good and really beautiful looking.
Oh yeah, cannot forget, while I only played in once, the original Halo: Combat Evolved was a blast to play. I played a pirated version of it on my ancient PC back in 2011, but it was really an amazing experience, and even to this day, despite it being a 2001 game, the graphics and the artstyle amazes me, and like, I remember the first time Master Chief's pod or whatever crashed in the planet, and I walk out, look at the sky, and that giant ring in space being seen from ground really blew my fucking mind.