I prefer to play multiplayer even though I enjoy zombies as well. I enjoy the competitive aspect of Call of Duty multiplayer and you dont really get that playing zombies.
I would prefer to play just zombies. Especially on World at War. When you get to the higher levels, it gets more challenging. For example, the ray gun stops being a one hit kill, and the deployables take much much longer to destroy zombies with. But having these challenges in the game make everything more interesting and fun overall.
I am a fan of both but I would say that I prefer multiplayer. I just love going online and facing other people. I always want to come out on top! However, I love playing zombies with one of my buddies. We are the perfect team.
Multiplayer all the way! Zombies can be fun with friends but it's never beaten Black ops 2 for me, I'm looking forward to seeing what they bring to the table for black ops 3 though! I've always preferred multiplayer as I like to go up against other players and test my skills I find it more rewarding
I love both however if I'm playing with people I know I prefer to play zombies. I find zombies a lot more fun but I only truly enjoy playing it when I play with people that I know. I sometimes find myself wanting to play Kino Der Toten.