Capture the Flag is marvelous. Sure the game objective is simple and done to death, but the gameplay feels so rich, and the engine adds nothing but depth to the game world - I'm absolutely loving it. I'm even digging DM and TDM, and I've never been a deathmatcher.
I bought Quake 4 expecting my stint to last no more than a few days, realistically. But here I am, playing it 2-3 hours every night since night of purchase.
I've even decided to upgrade for it, spending over "000 - I get my 7800gtx and nAbit KN8 Ultra next Wednesday... max frames in q4, w00t!
I played Q4 single player for like 15 mins. Single player First Person Shooters bore me to tears. The only first person shooter I think I've actually even finished is Thief... which is hardly a "shooter".