I voted Ushijima & Tendou because it seems like if they hadn't found each other, they would have both ended up without any close friends at all. That makes their friendship extra heartwarming to me. They're unapproachable and easily misunderstood by the people around them, so it's nice that they have each other.

Oikawa's in a similar boat, but he also intentionally keeps people at a distance. Everyone loves and respects Iwa, as they should. Kenma and Tsukki, ironically, are the type to naturally draw others to them without trying or wanting to. Bokuto is outgoing enough that he'd be okay without Akaashi, while Akaashi probably would have found someone else to attach himself to like we see at the end of the manga.