Well, we should decide what a bad set means. some of the sets that have been mentioned are bad looking, but historically correct, others are nice looking but historically useless. so i think the "worst" set should be one that is both inaccurate AND awful looking. and moreover, has no other redeeming feature. because even if some sets are inaccurate, they can be useful for conversions. again, some elder sets can be partially excused, at least they have some value as collectables.
so all in all, I can't tell you which set is the worst to me, I'll have to think a bout it a bit more