I'm with A Link in Time on this. Halo is up there with Zelda for me, so I don't have any problems with FPS games. I know competitive multiplayer is synonymous with the FPS genre, but that’s not something that I've ever really been into. I've dabbled in Halo multiplayer here and there, and while it's certainly fun, I associate Halo with the Master Chief and its storied co-op campaigns.
It's why I could never get into Call of Duty with its generic single player campaigns that were 2nd to its multiplayer offerings, which unfortunately became true of the genre as a whole. I used to love playing the Rainbow Six: Vegas games with my brother because they were so satisfying in co-op and for me personally, offered a richer and more grounded experience than the Call of Duty series. But now, Rainbow Six doesn't even have a campaign anymore.
Other than Halo though, which somehow fumbled co-op with its latest iteration, I find the Borderlands series to be a pretty consistent force for fun co-op FPS gameplay. But as far as single player games go, it takes a more special kind of game to grab my attention. Something with personality that's a game first and not just an excuse to ship a multiplayer mode to compete with Call of Duty.
While I may prefer my FPS games to be co-op, I can still enjoy Halo as a single player experience because the campaigns are inspired and the narrative is engaging. Also, there's just something really unique about Halo's gameplay and combat loop that makes it stand on its own amongst its competitors. Same with the latest DOOM revival which I've quite enjoyed for its fast paced and brutal combat that keeps you engaging in the action rather than hiding behind cover like most shooters.
I've also played the first Bioshock and while I wasn't exactly in love with it, it certainly had a ton of charm and atmosphere and to my surprise, felt very much like Borderlands in its gameplay and presentation. While it's not something I see myself revisiting, I am admittedly just trying to work my way through the series so I can play Bioshock: Infinite, which was the game that initially hooked me.
Which speaking of, I do have quite the backlog of FPS games that have stood out to me over the years that I haven't gotten to like Titanfall 2, Bulletstorm, the Far Cry series, Cyberpunk: 2077, and despite how I feel about games like it, Battlefield 1, which intrigued me for its World War 1 setting. Though, I have also played Battlefield: Hardline before, which I thought was pretty good for its change of pace from the typical militaristic stuff they usually do.