Which toon is almost useless without their zeta(s)?

Depends on how specific you want to be with the question really, if you want the toons with the biggest disparity from their non-zeta'd selves.
I would say good examples would be GGs metaloid monstrosity, without that the team would get countered by a lot more.
And 5s tactical awareness, without that 5s is just very conveniently killable