Single player is easily the first MW, going through that for the first time will never be beaten. MW2 came fairly close, but it lacked a lot of the really memorable set pieces to me, outside of the infamous No Russian level obviously.
Never really got into zombies in all honesty. The only two I've really played for any amount of time are the original Black Ops and Black Ops 3, so I guess I'll say Blops3 because I prefer the game itself. This doesn't count as zombies, but a shout out to the Extinction mode where you killed aliens in Ghosts; I actually played that more than zombies, and really liked it.
The best multi-player is almost an impossible answer, honestly. I want to say MW2 as it's just such a perfect chaotic mess that I still reminisce with with my friends about to this day. Everything was broken, it was so beautiful. But I also want to say Advanced Warfare. I fucking love Advanced Warfare. The air dashes, the guns were really fun, loved the whole exosuit vibe. You can dual wield wrist mounted heavy machine gun turrets, I mean you cannot beat that. TLDR; I can't decide between MW2 or Advanced Warfare.