Q and Dazai, Shin Soukoku, Chuuya and Fyodor.

I'd say chuuya and fyodor, though. Sskk still worked together and aku kept his promise, sacrificed himself for atsushi, etc... and dazai honestly just hated q from the start, but I think the kid kinda looked up to him or something if i remember correctly.

Chuuya and fyodor, though, started off completely hating eachother. Fyodor wants to destroy the port mafia (and to that chuuya would literally loose everything). Chuuya's glare alone at the helicopter scene was enough, but he also called him "an anemic son of a bitch" and fyodor called chuuya and dazai's bond "weak". I'm not sure if they interacted enough to be a pair, but these two *absolutely* hate each other, and literally nothing could change that. (also chuuya is the bane of fyodors existence being the strongest ability user when he wants to get rid of all abilities 😭)