Squall always has to have Diablos or Shiva
Rinoa has Carbuncle and nowadays Alexander
Quistis and Selphie are rather interchangeable, with Quezacoatl and Cactuar
Zell for Ifrit and Brothers
Irvine gets Diablos if I know I shouldn't give him to squall and waste the hit junction.

The rest don't matter so much really, so I assign them by vitality junction first (carbuncle, shiva, and then there's one other that you can junction to vitality with:O) and then filling in the rest of the holes.