Final Fantasy IV
Best: The introduction of ATB
Active Time Battle became a Final Fantasy staple with Final Fantasy IV. The battle system merges turn-based strategy with a frantic, real-time combat that both spiced up fights and made them more exciting, but also added a sense of urgency. Square would go on to riff on this idea over the years before eventually pivoting hard into a straight-up action game, but Final Fantasy IV kept the series fresh in those early years. Even among some sweeping changes, the series would continue to use it for several years.
Worst: It does feel a little simple now, though
Final Fantasy IV was the battle system blueprint for an era in the series’ lifetime, but because games like Final Fantasy VII and XIII so effectively iterated on the idea of ATB, the fourth game’s combat feels a little less impactful these days. Hats off to the OG, though.