I tend to go partially by gameplay and partially by story for Birth by Sleep, so I go with the suggested order of Terra, Ventus, Aqua. Terra tends to mostly favor offense and is a Mighty Glacier for the most part whereas Aqua is more of a mage fighter and cannot take a hit. Ventus is the middle ground here since he is fast and agile with little defense and also has a relatively equal emphasis on both physical and magical attacks. It also makes sense for me to do this route since you only gain the Final Episode after beating all three and the Secret Episode after that. Since those two focus on Aqua, it makes sense for her story to be third so that I can pick up directly where I left off.