Which personality theory do you think offers the best explanation for how an individual's personalitiy develops?

Erikson's stages of psychosocial development.

also, just understanding the role that the Id, the ego, and the superego play in personality development is beneficial in many ways.

having insight on what self-efficacy is, as well as the difference between a type A and type B personality, lotus of control, etc...

Understanding Identity and Personality disorders is worth looking at, too.