This is tough. I could go with a bad regional show like Blame it on the Movies (which is truly the worst musical I've ever seen), or a bad Broadway/West End show that had little-to-no redeeming qualities such as Thou Shalt Not or Notre Dame de Paris (predictable), but there is one show that is actually a musical theatre standard that I have tried to like, but have never been able to enjoy:
Annie Get Your Gun
I almost said Follies, which is a close second, but the score for Follies contains more songs I enjoy.
"What can you expect from a bunch of seitan worshippers?" - Reginald Tresilian