I personally like Counter-Strike (fy_ maps only though, like fy_iceworld or something. I don't like to play for objectives.)
I think UT2k3 kicks ass, and so does UT.
I am planning on buying UT2K3, HL, and Quake 3 Arena sometime. I don't own a legal copy of HL so I am gonna pay the massive 20 bucks to get it.
Also, what are your favorite HL mods? Opera is a cool one if you can get good at it. I heard DOD is good, Firearms is good, but I haven't played either.