The anime ending that most disappointed me was Amon: Apocalypse of Devil-Man on account of actually having expectations that were not met based upon foreknowledge of the manga's conclusion and the quality of the other two entries in the anime movie series based upon said manga rather than merely having a negative opinion of a particular ending.
After all of the build up to the climax, all of the grueling battles with the worst demons that Hell had to offer including Akira's own literal inner demon(the titular Amon), after all of the sacrifices made by Akira Fudo and his allies,when he finally confronts his nemesis, this happens.
As far as disappointments go, it scarcely gets any better than this.
After all of the build up to the climax, all of the grueling battles with the worst demons that Hell had to offer including Akira's own literal inner demon(the titular Amon), after all of the sacrifices made by Akira Fudo and his allies,when he finally confronts his nemesis, this happens.