I'm really not fond of romance being present in the series in the first place. I much preferred how it was in the first book where a she-cat would mate with a tom not for romance but just as a way to make the clan stronger. It seemed like they just mated with whoever was around at the time, in a way. When I read a book about cats, even if they're talking cats, I want them to act like cats. I don't want them to act like dramatic high schoolers in cat bodies.
Though I have a particular distaste for Fire/Spotted, Crow/Leaf, and Crow/Night.
Even as a kid Fire/Spotted confused me because Firepaw's crush on Spottedleaf didn't even seem like a romantic one, it mostly just seemed like he thought she was attractive. And then she dies and the Erins write it like they had some intense romance going on.
Crow/Leaf was just unpleasant to read about and had a huge amount of focus in the book. Also Crowfeather is just unpleasant all around and I really wasn't fond of Leafpool abandoning her duties like that. I'm not fond of a warriors abandoning his/her duties, but duties of a medicine cat are even more vital to the Clan and could put the entire Clan in danger if they're neglected.
Crow/Night for reasons others have stated here. Crow's just awful. Nightcloud really deserved someone who would be kind to her and Breezepelt deserved a father who wasn't abusive.