Cough cough
I'll say right away that if the route is written well, I'll still enjoy reading it. But yeah, there's probably no chance the character will become one of my favorites.
Yanderes. I've already said this in other threads, but I really do hate them in like 80% of cases.
Childhood friends. The only exception I can think of is probably Gekkamaru from Nightshade, whose route I actually really liked. But generally, I'm not a fan of childhood friends who are already in love with the heroine. They tend to feel kind of pathetic to me, especially when you think about how they're still in love with her in all the other routes too, they just never had the courage to confess.
Boring adult men. Tooya Masamune (Norn9), Serigano Manatsu (Ayakashi Gohan), Kuroba (Olympia Soirée). I usually just feel like there's no spark in their routes. They're too calm for me, I guess. I don't even know what the right term for this type of character is, but yeah.
PS: Genki characters and playboy/flirty types could probably also go on this list, but with them it's really 50/50. Sometimes they actually end up being some of my favorite characters.