TNG season one is not the worst season at all, it has some real gems in there. It does have a terrible, terrible pilot episode though I won't deny that.
I am going to say Voyager S7.
Opinions vary. TNG has Code of Honor, and that, outside of ENT's Cogenitor, is the worst Trek episode I've ever seen. Even the TNG cast didn't like that episode. I only got half-way thru season one, though. I do hope to find those gems in the second half.