No More Room in Hell the long awaited Source zombie modification.
My main issues with this is its awfully slow, no while the newer patches fix the issue of stamina running out all the time (with the inconvenience of there being no stamina bar to tell you), I still find things I hate like how slowly it takes to aim the revolver like your character is taking his time. The lack of any tags on servers so you don't know which one is running realism mode and thats the only mode I wanna play otherwise it takes multiple headshots to take down zombies which makes the rather limited ammo & slow melee swings all the more painful. This would on its own be a deal breaker for me, but I think that the best zombie games/mods are probably open world kind because it opens up all kinds of encounters and replayability. Survival is emphasized in these settings rather than the "speed" like in these kinds of maps, the slow gameplay kind of weakens the point of getting through the objectives fast.
Especially when you start running into fast moving zombies.
I rank this higher than most mods because it has alot of work done on the maps/weapons and I like the zombie/character models. It needs more characters though and its barely halfway there to being a potentially good mod for me to get into.