I can't think of anyone and those of you who can most likely just have an ax to grind with a band you are sick of hearing praise for.
I mean if a band had a particularly bad singer during the 1970's, either he was quickly dismissed from the band or we never heard of them to begin with because they didn't get signed.
Since I don't like to be a "downer" who pees in the pool, though, I will probably just say the guy who was in Journey for the first few years before Steve Perry joined. His name was Robert Fleischman. If he was anything special, why the hell didn't he last?
Then you also have a guy (Al Atkins) who was in Judas Priest before Rob Halford and I'm guessing he wasn't anything special because Rob quickly replaced him.
That's it, all I can come up with.