As RTS I prefer playing Empires mod for HL2.
As Zombie game I'm waiting for 18 of november for release of Left 4 Dead, and until that I'll just play ZPS.
ZM is pretty dead from the side of developers.
Honestly, it's gay. Fun for a couple of hours, then you notice how repetitive it gets. Same maps, same technique. Different types of zombies, traps, and the ZM needs resources. You have to find weapons and ammo. I stopped playing ZM after a while. I now play Zombie Panic!