Only played Golden Deer, but...
Claude x Petra would throw the best party Fodlan's ever seen at their reception. Then they would both go sleep in a tree.
Marianne x basically anyone. From every support I've seen with her they lay it on early and they lay it on thick. Special shoutout to the Marianne x Hilda pairing that will never be canon.
Ignatz x Leonie. They're just very supportive of each other. Also, as much as I like Flayn hounding Felix to paint her better a (spoiler redacted) will never not be kinda weird to me.