There is no single "best" party; it depends on playstyle. Here are three strong archetypes.
Strengths: Versatile, easy to learn, covers offense/defense/support.
Strengths: Melts enemies fast. Weakness: Lacks dedicated healing/support.
Strengths: Excels in long battles, status effects. Weakness: Lower damage output.
Experiment with Paradigm Shifts, Stagger, and Crystarium to optimize.