I'm an Unreal Tournament fanboi, so I'm going to come out and say "Unreal Tournament" even though Unreal Tournament III is unpolished and has an icky user interface (excellent game play though).
Why? Imagine being able to download and install user-created game types. Imagine being able to download and play user-made maps! Now play them all online against other people! You can join clans and play team games in leagues. Make friends, join discussion forums, discover new communities. Heck, you can even learn to create your own user-made maps. (I've authored a few myself and have a UT3 CTF map in the oven right now.) I've been playing since 2001 and by far, it's my favorite game.