My favorite is still good ol' Quake 2. I been playing it now for over 2 years, and I'm constantly improving. I think that's one thing that makes a game fun, seeing yourself improve, as long as you don't hit a wall, you just wanna keep pushing yourself further and further. I honestly have never played Counter-Strike though, so I can't comment on it, though it's about all I hear about. I only played single player HL. Quake 3 is just not that great. UT is fairly fun though, but for me, for good ol' deathmatch, it's hard to find better competition and fun out there besides Quake 2. Oh, and for a game being "online only", there's always LAN games. But for Quake 2, single player blows, so Multi is really the only option for fun.