I've been meaning to try out Zombie Fortress, but I can't find any servers with the mode.
I usually go on Prop Hunt, Control Points, and Saxton Hale-based servers. But my favorite mode is a modded Control Points not sure if it has an official name. Basically, all we do is take a half hour out of our lives to kill each other as much as we can. There aren't any actual control points, as the loading screen would suggest, so it all ends in a stalemate everytime, making it a fun and challenging map also helpful for getting any items because of all the dying.