I also really disliked Yellowfang's Secret, but everyone beat me to it on the reasoning!
However, my least favorite Super Edition is Bramblestar's Storm. Maybe if I reread it, I'll like it more, but it really quashed my interest in Warriors as a whole for months because of how disappointing it was to me. Brambleclaw was, until that point, my all-time favorite Warriors character. I was happy with the kind of character he became by the end of the series. I was glad for his development by The Last Hope, and he really earned his spot as leader.
But he has not one, but two wacky relationship developments in that book. The first comes with Jessy, who is otherwise a pretty okay character, and I have no ill will towards her, just the really forced nature of her “i could love you” relationship with Bramblestar, which is kinda weird. The other thing is that he apparently has to get back with Squirrelflight? for some reason? like because they're the parents of Lion/Holly/Jay, they have to be in a relationship. They can't just be separated and on good terms. it actually happens sometimes, ya know? it just bugs me that they felt they needed to hammer in their rekindling romance after all their time apart and coming to terms with everything.
…i just basically disliked Bramblestar's Storm because i didn't want to read a love triangle and that's what about 50% of the book seemed to dwell on.