I honestly like Toy Story 4 more than I like 1. If there has to be a “worst” in the series, it’s actually the first one, but it was so revolutionary that it deserves its recognition as a moment in time.
But again, there are no bad ones.
From what i heard about 5, 4 is the worst of the series. But i'd argue the real bad thing people can say is the fact that a 4 even happened knowing 3 was really the perfect ending.