no way i can decide, so here's a bunch of games i like
doom - still fun, so influential
hl1 - still the best hl, love the survival/mystery atmosphere
turok - showed FPSes could work well on consoles, cool death animations, still feels great to play
ut99 - gibs, maps, music, and gibs
r6 ghost recon - first long distance realistic shooter that really worked IMO
r6 raven shield - best close quarters realistic shooter (unrecord has potential to be better)
goldeneye - MP was so fun and cemented that controller rumble here to stay
solider of fortune - set a precedent for gore
battlefield 2 - something about it, legendary demo
dayz - tons of jank, but tons of unique experiences too (coop)
project reality - loved the way the aiming worked in this game (let the gun "settle" = better aim, no indicator to let you know)
stalker - bit janky, but cool survival atmosphere
just going to stop here