By far, my all time favorite game is the original[/u] Unreal Tournament (the one released in 1999). It is also the greatest online multiplayer first person shooter of all time. Not Half-Life, not CoD, but UT.
I've played it for years, managed a capture-the-flag clan, and I played CTF competitively all the time up until I bought Sins of a Solar Empire in mid-March. I also authored five custom CTF maps for the game, some of which were played on public servers and in clan-match-style pickup games (PUG) matches. It has been, by far, the most engrossing game I've ever played and it's managed to hold my attention for over half-a-decade. It's all about online multiplayer, custom mods and maps, and freedom.
UT 2004 was also excellent in its own way though some aspects of its gameplay were inferior to UT99 (the floaty-dodgeyness and its resulting emphasis on hitscan).
Note that UT3 is a POS and an abject failure and is not representative of what the real UT game is about. The UT3 gameplay isn't too bad but everything that surrounds it is awful (the user interface, the server browser, having to log into Gamespy, how it handles downloading custom content, the file structure, etc.).