That's a tough one because of what's needed for a team to be succesful, which requires a variety of skills to adapt to different situations.


You need someone who can consistently hit for damage (melee)
You need to be able to heal yourself (healer)
You need to be able to exploit weaknesses (nuker)
You need to have adequate defense (tank)
You need to be able to improve your party's defense (buffer)

Some combinations are a pretty natural choice. Tanks and melees are often the same thing, simple because melee jobs get better armor. Healers and buffers are also the same thing in FF for the most part (white mages).

So with a 3 person setup...

Character 1: melee + tank
Attack
Cover
(insert defense or strength boosting ability here)
Item

Character 2: white mage
Attack
White Magic
Defend
(other buffing ability such as Cheer)

Character 3: black mage + summoner
Attack
Focus
Black Magic
Summon