Pretty sure the first two guns i can think off of this question would be the Havoc and the Sirin from BO7 and BO6 respectively.
The Havoc is more or less a meme gun that can also get some good value in Multiplayer off of camping chokepoints, and can be useful with the prestige magazine if you are able to spam it at a group, but in Zombies you can legitimately delete hordes off of existence with only a couple of nades, and the prestige mag also helps a lot.
While the Sirin is an okay-ish SMG secondary that can work if you are in a pickle, it turns into a genuinely OP gun in Zombies due to both the damage output you can deal to hordes and elites, and also the ammo efficiency the gun has due to it having a considerably lower ROF than actual SMGs.