Rezurrection - awesome map pack if you ask me. except Moon, which is a new one, the others were already released, prevoiusly for other CoD Games, but i really recommend them. and together with all those cool zombie maps, you also get the Soundtrack and a Zombietheme for free! (At least with the PS3 version, dunno about 360)
edit -
aah, misread the whole thing xD! i'd go with the DLC that contains 'Call of the dead' (If it is possible to buy it without getting all the other first) since i think this is, by far, the best zombiemap out there.