@timhansenYT I have to say the Sliquifier because it was very fun to use and it is one of the best wonder weapons of all time. When it was prepatch it was the most fun because I like to exploit broken mechanics in games. Too bad its on a map that gives me depression. :)
@timhansenYT If we are talking zombies specific then I would have to say bo3, due to mechanics, movement, maps, chronicles, and mods. Bo2 was amazing because it had ORIGINS, MOTD, and tons of glitches which was fun :).