Good 'ole fashion Quake III Arena for me, with the Generations Arena mod installed. I really wish they'd update the mod. It's supposedly still in development, but I haven't seen an in years. At this point, it still needs weapon animations!
Quake Live is nice, but the lack of custom maps, models, skins, and game-modes is a huge downfall for me. Plus, having to pay to host matches hurts. I'd rather see an updated Q3A on Steam, kind of like they did for Xbox Arcade... I'd want it to be fairly compatible with the original title however. No need to remake all of those awesome mods and maps!
I have a Killing Floor and Minecraft server that my friends and I play on fairly frequently as well.
Been trying to get into Payday 2, but no one I know seems as interested in it as they were in the first title.
I still wish I could get enough people together for Team Fortress 2.
Yeah... I hate playing with strangers. :P