Well he did ask for recommendations so I was hoping that he wouldn't be completely burned out yet! Everyone has gaming ruts!
But yeah, I grew up with Doom II, the Quakes, Duke Nukem 3D and then Half-Life (and bazillions of mods. I probably put as much time into Sven-Coop alone as I put into HL) and started playing CS while it was in beta. I used to think that I loved the FPS genre because I had really pleasant experiences these games, but whenever I tried any new ones, I ended up putting them down in disgust. I didn't feel like the newer games did anything better than the previous ones.
Look at level design, for example. How many modern games can parallel the excellence in level design of the old goodies? Very few.
Graphics for me are moot because I care for them very little. Gameplay above everything.
Anyway, after having tried many, many other FPSes over the years CoD:WaW was what resuscitated the genre for me. The SP experience is mediocre but the MP is fantastic. I've ranted about the excellence of this game on this forum on many occasions already, so won't go into them again.
I've given fair tries to all the other CoDs release after WaW (I often have friends come over who are CoD players so I got it to play with them really [hot seat is awesome]) and they've been nothing in comparison to WaW.
Honourable mention here goes to HL2 - that game was pretty good but not good enough for me to play it more than once. I still haven't played EP1+2 even though I've owned them pretty much since release. They are ranked pretty high in my backlog though.
I think the major turn-off for me in modern FPSes is the lack of satisfaction that you get from killing. It simply isn't there. For example, the weapons in HL2 are so gimpy (and I'm also not a big fan of the Source engine. It's ok.) Sure the graviton gun is nice, but it's gimmicky and too easy. It doesn't compare to shooting a dude in the face in CS, or railing someone to bits in Quake II. And games like whateverthatonewascalledwhereyouhadtokilllegionsofdudesinhorrendousways simply make it so easy that it's no longer special, it's the rule rather than the exception. You don't get the satisfaction from pulling off a skill (or even a lucky) shot.
On top of that, these games don't have the backbone in solid and refined game mechanics. All of this means that people end up dropping them quite soon and moving onto newer games. They cannot carry their own weight and thus the community ends up weak, leading to lack of development in the multiplayer experience and a poor experience overall due to a lack of players.
WaW has solid mechanics and it offers satisfying gameplay. Also, map pack 3 introduced some of the best maps I've ever played in mutliplayer (admittedly, what came before was rather poor).
Oh and I've written enough, cannot be bothered to explain why I don't like the MP experience in the other CoDs. They will simply be forgotten once the next instalment comes out because they are simply unpolished turds hastily churned out from Activision's asshole.
edit: oh and I also thoroughly enjoyed Doom III, don't see what people didn't like about it.
FarCry and Solider of Fortune were also great, but I don't think that either one has aged particularly well. The latter has gone especially sour.