Black Ops 1 zombies is pretty entertaining and personally nostalgic, playing as JFK/Nixon/Castro is so campy and definitely worth experiencing at least once. I never had the DLC for the moon base but people seem to hold that one in high regard. Same goes for Tranzit in Black Ops 2.
I think the general consensus is that the single player campaigns of Black Ops 1 and 2 are the best in the series, while the later Black Ops have better zombies modes? Hell there's even one with Jeff Goldblum which I find pretty funny.
I think it depends on what you're looking to get out of zombies, there's a whole subculture dedicated to pulling off the elaborate Easter eggs that might be worth looking into.