If you're on console, Black Ops 2. The maps aren't my favourite, but IMO the core zombies experience was perfected and it's been all downhill since. Polished experience with a solid gun roster and nice difficulty. Plus there's maps for nearly every skill level. Just wish there was more than just Nuketown and the sub-Tranzits for those of us who want a pure survival experience.
If you're on PC, WaW or BO3. WaW hss a massive back catalogue of custom maps, plus is one of the few CoD games that's reasonably priced on Steam. BO3 requires you to pay a lot more, but is a much easier source of playing modded zombies. This is just my opinion, but I'm not a big fan of unmodded BO3, guns are uninspired and feel awful, and gobblegums are a fuck that never should've been added. And then objectively, WaW is super buggy, and has a fit if you alt tab, but, subjectively, the simpler custom maps (not to say they can't be complex, Leviathan was a WaW custom map) give off a nostalgic vibe.