I spent ages looking into this. Apparently there are optimum setups so that each character has access to 3 swiftness unlocks. Then there is taking into account what can be considered canon classes for each character.
I eventually went for:
Vaan - Shikari / Foebreaker
Ashe - Time Battlemage / White mage
Penelo - Monk / Black mage
Fran - Red battlemage / Archer
Basch - Bushi / Knight
Balthier - Machinist / Uhlan