Puella Magi Madoka Magica. It relies solely on one or two moments of shock value to sell it's mediocrity and that is it. It's badly written, poorly paced, predictable despite trying to "surprise" with its blatantly grim tones, doesn't bother to develop its cast at all - it kills them off before the viewer realises that they have no discernible personality whatsoever - has an absolutely hideous art style, and barely qualifies as a magical girl anime. It's not a deconstruction of the genre either. Madoka is the worst show I have ever had the misfortune of sitting through, and the fandom only amplifies my irritation with it. I know the fandom is not the fault of the show, but that doesn't excuse the show's poor quality, and it's worth mentioning.
Eureka Seven is another show I absolutely despise, for its awful art style and meandering plot that never really goes anywhere despite initially showing some promise...this can also be applied to Gundam Reconguista in G, although that never really showed any promise. Darling in the Franxx and Guilty Crown are in a similar category, only with a less vile art style and a much, MUCH more aggravating cast.
These shows that initially show promise but ultimately decide to squander it by never developing characters or themes, or just never push the narrative forward, are what irritate me the most, as they make promises they never deliver on, and of course I watch them to the end thinking that all this garbage will pay off. It never does, though.