First, I would like to say that ConcernedApe did a wonderful character design job with all the NPCs in Stardew Valley. This can easily be spotted by all the posts (in this thread and in others) where one tries to guess the psychology or story of the NPCs solely based on the text lines they say in-game : you don't see that with poorly designed characters.
Regarding the original question of the worst NPC, I will rule out Morris because he is designed to be loathed.
I can't think of a NPC that I really dislike: they all have their qualities and flaws. I can bond more easily with some of them for personal reasons, but I guess everyone here could say the same, but not always with the same NPCs.
Thus, I don't have a worst NPC, only one that I like less than the others and that would be Gunther. Come on! A german name with a cowboy look? He must have done terrible things in a previous life...
However, I'm curious why so many people dislike Pierre. I did not have the opportunity to see his new booth in patch 1.5 yet
(and I think this is a pity he got one during Dance of the Moonlight Jelly festival which is my favourite one: so poetic) but I wonder why many think of him as disloyal and greedy. It seems people do not remember him always being that way:
LRangerR said:
Like i've said before, I really dislike how they've changed Pierre's character over the years. He didn't always used to be that way.
Has he changed that much over the years/patches? (if this is too much out of topic, I can open a dedicated thread)