Have only played Exo zombies, Bo3, Bo2 and Infinity Warfare.

How I would rate them;

4: -Infinity Warfare zombies - nothing wrong with it or anything, fun gameplay. I just liked the others more. It also only has 2 maps so far so obviously it's not fair to put it above others anyway for the time being.

3: -Bo2 Zombies - Had a nice mix of relatively tough maps like Transit and some things to do when you're burned out like playing grief(Miss that!). Fun times.

Tie for #1:
-Bo3 Zombies - From the atmosphere and feel you get playing on Shadows of Evil matched by few followed up by an instant classic in Der Eisendrache, this game kept me playing TOO MANY hours that should have been spent being productive.("I'm killing zombies babe I'll do it later!")

-Exo Zombies - This is the game that turned me into the zombies huntin fool I am today. Not many people get past the titles creator (not Treyarch) and immediately wrote this game off.. Do yourself a favor and play this zombies mode starting from Outbreak(the most boring and basic map out of the 4), to Descent(the final map in which you have to fight your old friend you fought alongside for half of this exo zombies experience). The best map that I still play today (my opinion) is Carrier. OMG you haven't lived til you shot the LZ-52 Limbo. It shoots out a horizontal lazer beam about as thick as a ruler but extends all the way from the left side of your screen to the right. Of course with this unique gun there were tough unique challenges such as diffusing bombs that would appear with guys with guns guarding it (use the limbo!) and last round 20 a Goliath robot will join in protecting the bomb from being disarmed.
Another freaking awesome thing that was introduced in Carrier was teleport nades that you could buy off the wall. Wherever you throw them that's where you are. Need to stock up on teleport nades? Just TELEPORT to the TELEPORT nades, that simple. So many clutch revives are to be had from these. Also a little known tip was you could use the teleport nades to teleport inside the Goliath robot guarding the bombs and it will insta kill him!

Sorry played carrier too much had to tell you people what your missing! Think I'm still ranked around top 60 in Carrier Career on the PS4, not that that means anything.