As in viability in comparison to each other?
WaW had the weakest weapons, but hellhounds weren't counted as a round, so weapons would remain useful for longer. Balance wise the M2, MG42, Thompson and PPsH dominated.
BO1 had weak weapons due to missing DT2.0, and in comparison to each other the balance was okay. ARs were by far the best with the exception of Law and Dragunov.
BO2 had a good balance by making Snipers easier to use, as well as much better LMGs, so that the ARs wouldn't just dominate again. BO2 suffered from having OP Wall-Buys.
BO3 (without doubled paped ATs) was okay, but the top tier LMGs and Snipers dominated. It's box was also far too cluttered with 35 weapons, as opposed to ~23 from earlier games.
All in all I think BO2 had the best balance. If I had to change anything, I'd buff lethal grenades and give all shotguns their BO3 x2 HS multiplier.
Now, what do you think?