I'm going to answer by interpreting the question to NOT want the "worst" endings, just the ones that most disappoint -- and to disappoint, there needs to be some sense of letdown due to higher expectations. So the top ones I'd name include...
Matrix Revolutions: So the whole thing comes down to just one more fistfight between Neo and Smith in the real world? And then Architect just agrees to make peace with humans? The whole film was slightly disappointing, but the fact Neo does very little for most of the film and then resolves his and humanity's arc by just having a fight with the same enemy series, followed by the machines just suddenly agreeing to peace, wasn't worthy of all that came before. This series started so brilliantly, and had a sequel that shocked with a revelation turning our sense of reality on its head yet again, so it was all the more sad to see it end like that.
Contact: Finally, the alien planet! Finally, humanity's first contact with other intelligent life! Finally, we meet the -- oh s**t, they're disguised as her frigging dad? Added bonus letdown -- oh look, the backstabbing hunky religious dude made his point, the scientist's mind is blown, she has to embrace faith, OMG! I loved this move right up to the very end bits mentioned here. I was so excited when I heard the film was coming out, and it was so great all the way through that when the end came, my jaw must've dropped wide open in disbelieving anger.
A.I.: A film that I think is overall pretty flawed anyway, but considering the talents and minds behind it and the quality of the production, it led up to an ending that was surprisingly much better than the rest of the film ... only problem is, the ending that was so much better and so impressive and that made the rest of the film worth watching wasn't the real ending after all. What impressed me was when he's on the bottom of the sea, stuck forever, wishing to the fairy to turn him into a real boy. That was beautiful and tragic and wonderful. Except then it's followed by another ending, where he's found two centuries later and rescued by advanced evolved artificial life forms on an Earth without humans, and these new life forms recreate his old home and bring his mother back to life so he can have one perfect day where she loves him again. I know there were themes about why he cannot become a real boy and the irony of humans having wanted to make artificial life while the machines want to understand human life etc, but the fact remains that I think the better ending and one that much better reflected the themes of the film and brought the narrative to the proper close was the one under the water, and it was disappointing to think the film had just had such a bold and perfect ending only to see that it was going to try for sentimentality instead.
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