These are the ones I know I'm doing:
MaMUxLissa for that MU!Owain and Lissa!Morgan. Both make extremely strong magic units late game imo.
DonnelxNowi tanky and easy to train dragon with sol, pavise, and armsthrift? where do i sign.
VaikexSully I know VaikexNowi and DonnelxSully is the usual way to do things but I've always had more success with VaikexSully not sure why.
GregorxMiriel Armsthrift on my witch man means Sorcerer with infinite Mires which was really helpful on normal lunatic. Tell me if that's not as good on Lunatic+
KellamxPanne Simply the best use of Kellam I've found.
FrederickxCherche Intoducing the new friend Gerome with crazy defensive and physical boosts.
The pairings I'm less sure on: This is just off the top of my head if there's a pairing that's good that I'm missing feel free to tell me. (also if I forgot a mother my bad please tell me I'm sick so not thinking completely straight.)
GaiusxTharja
VirionxTharja
ChromxSumia
ChromxOlivia
LibraxOlivia
StahlxSumia
Lon'quxCordelia
StahlxCordelia
GaiusxCordelia
HenryxMaribelle
LibraxMaribelle