Plenty of people woon and awe over the talents of well recognized figures like Yoshinori Kitase (co-director of FF6; director of FF7 and FF8; producer of FF10 and FFX-2) and Tetsuya Nomura (character designer for FF7, FF8, and FF10), the two men most prominently credited with the direction the Final Fantasy series following the scaleback of Hironobu Sakaguchi's direct involvement after his promotion to the top of the Squaresoft developmental ladder. Others, owing to perhaps a more old school flair, focus on the considerable skills of Sakaguchi himself, the man who created the series and directed the first five games.
But for my money, one of the most significant figures in shaping the Final Fantasy series is also one of the most underrated: Hiroyuki Ito. He's long toiled away in relative obscurity, but he's been incredibly significant, influential, and as I've lately mulled over my preferences within the series, I've come to recognize that he's probably one of my favorite things about Final Fantasy.
Ito is, above all other things, a master and maven of gameplay--and as important as good and original gameplay is within an RPG, this is nothing to ignore. It was Hiroyuki Ito that created the Active Time Battle system that has since become almost synonymous with Final Fantasy combo; after first appearing in FF4, it's been featured in FF5, FF6, FF7, FF8, FF9, and FFX-2 (based on what we know, FF13 is also supposed to use a modified version of the ATB). Ito then went on to serve as one of the lead game designers for Final Fantasy V, which meant designing the most successful implementation to that point of the series' legendary Job System (complete with an in-depth skill learning and equipping system), before he hit his big break within the team.
As Sakaguchi stepped back his hands' on role, he tapped two men to lead the Final Fantasy series: Yoshinori Kitase (who these days is the more widely recognized, and has risen to greater prominence within the company) and Ito. Kitase and Ito both served as co-directors on Final Fantasy VI, one of the most critically and popularly praised RPGs of the pre-3D era. In this capacity, Ito was assuredly a key figure in the designing of the liberating Esper system that served as a model for FF7's materia system. After his success on FF56, Ito moved temporarily away from directorial duties to focus on game design again. At this point, he served as the lead game designer for both Final Fantasy VIII and Final Fantasy Tactics. For the former, he would design the complex but highly original and freeform junction system, while in the latter game he meshed FF5's Job and ability systems with the core elements of strategy RPG gameplay.
Ito wasn't done with directing, though. After wrapping up his design work on FF8, he went on to direct Final Fantasy IX simultaneously as Kitase's team was working on completing the preceding title. It was Ito's first solo run as a director, and I believe it was a great success--of the post-SNES Final Fantasies, which are sometimes criticized for too much story and cutscenes (in the case of FF8 and FF10) or too much gameplay (in the case of FF12), the 9th game had perhaps the finest and most balanced blend of the cinematic story and customizable gameplay. After FF9, Ito was the lead game designer for Final Fantasy XII (for which he created the Active Dimension Battle system, a modern-day challenger to his old ATB) until Yasumi Matsuno departed the project. At that point, Ito moved up to again serve as co-director of a major Final Fantasy title, this time with artist Hiroshi Minagawa.
Director of FF6, FF9, and FF12 (three of my favorite games in the series); creator of the ATB and ADB; refiner and designer of the Job System; father of gambits, junctions, and item-based passive & active skill acquisition. It's undeniable the huge role Hiroyuki Ito has played in shaping the Final Fantasy series and in contributing to its quality and success--it's certainly, if nothing else, greater than the role played by a mere character designer. If only this incredible talent got the credit he deserved.