Wagner - Tristan und Isolde. Everything in it is very well done, but it makes me feel like the composer is trying to blatantly and shamelessly manipulate me, and my brain naturally rejects this. It feels like a bizarre form of "re-education": You Are Feeling Longing Now. You Must Feel Sexual Arousal For The Next 37 Bars. The Dominant Seventh Pedal Will Continue Until Emotional Response Improves.
Mozart - String Quartet in B-flat major, KV458. The only Mozart piece I know of that actively annoys me (apart from some of the more overplayed numbers from the London sketchbook).
Beethoven - Symphony No. 9 in D minor, Op. 125. The first three movements are boring, and even feel like the composer was bored; that he was still trying to write in his "middle period" style but had run out of ideas. Things pick up when the singers enter.
Chopin - Polonaise in A-flat major, Op. 53 ("Heroic" or "Carpal Tunnel Syndrome"). I am actually not alone in this, the rest of my family members also hate this piece. I think it may just be Chopin's style not doing well in pieces of an unambiguously triumphant cast; it feels crude and violent when it's probably not supposed to. (I love its counterpart, the Polonaise in F-sharp minor Op. 44, which has almost the same exact structure, but in which the crudeness and violence are completely intentional.)
Shostakovich - Symphony No. 10 in E minor, Op. 93. I guess it's not terrible, but people regularly point to this as the composer's greatest work while apart from the scherzo (which is indeed very fun) the rest of it feels like a slog. In live performances I find myself half asleep up until the tam-tam crash in the last movement, which signals "almost time to go home".
I have actually listened to all of the Strauss tone poems with the assistance of my university library's Strauss Complete Edition scores, and nevertheless remember none of them, so I suppose Heldenleben may be terrible or maybe just unmemorable but it didn't stand out as particularly either one compared to his other works. (The only one I remember is Till Eulenspiegel and that's because we had to analyse it in a high school music theory class.)
I don't mind the Mozart Requiem, though it's not his best work by any means. But honestly in general, for most of everyone else's choices (even the pieces I love), I can definitely understand why someone would hate them. No real WTF moments in this thread from that perspective.