Arguably, the most popular title in the entire series, boasting a highly customizable character system and growing trend toward a colder and darker futuristic industrialized setting, replacing the traditional historical Medieval backdrop so common in earlier prequels.
Indeed that new materia system brought forth high degrees of character customization that surpassed earlier class change and esper systems.
The newer industrialized setting hallmarked Square's leap from Nintendo to Playstation, from knights and kings to robots and machines.
Square had prepared for the future, and Final Fantasy VII explains why.
This site offers my personal commentary on the various character and materia in the game, plus a look at Square's history.