This is blasphemy but my two favorite shooters of all time are on the Playstation 1 due to the musical score and sound effects that aren't on the PC versions.
The original Doom game. We spent a lot of time figuring out how to LAN multiple computers together to make that work.... then we'd play for HOURS once it was all set up.
Well, it's hard to pick an absolute so I'm gonna go with the Half Life series (and mods) and the Battlefield series. No other game(s) have been me as much value and entertainment.