This is a sort of guide which I follow when pairing up the characters, some I've set in stone, others bend the rules a bit, some I haven't agreed fully upon yet, just wanted to share this and also get your opinions.
Scout:
Primary - Tracer
Secondary - Genji
Tertiary - Lucio?
Soldier:
Primary - Pharah
Secondary - Soldier 76
Tertiary - N/A
Pyro:
Primary - Mei
Secondary - Zenyatta?
Tertiary - N/A
Demoman:
Primary - Junkrat
Secondary - Brigitte
Tertiary - Reinhardt
Heavy:
Primary - Zarya
Secondary - Roadhog
Tertiary - N/A
Engineer:
Primary - Symmetra
Secondary - Torbjorn
Tertiary - D.Va, Winston, Bastion, Orisa, McCree?
M