I already explained my opinions in detail. Garnet was the only one of the four that served an actual purpose in her game's story besides being the love interest for the hero.

Aeris - Her entire purpose was to soften Cloud's heart and to summon Holy. Beyond those two things, her character didn't evolve or expand at all.

Rinoa - Her entire purpose was to soften Squall's heart and to become Edea's replacement as the Sorceress. However, her transformation into the sorceress did little to alter the course of the story overall, considering Squall and the rest of the party treated her as though nothing had happened at all.

Yuna - Her entire purpose was for Tidus to obsess over her. In the end, her entire quest turned out to be a sham, so her necessity to the story was nullified.

Garnet, on the other hand, served as a character that evolved and expanded much more than any of the others did. She went from being a snotty brat to being a self-sufficient leader over the course of the epic. Her purpose in the story was actually more pivotal and emotional than Zidane's throughout the majority of the quest. She had value and importance out the wahoo, whereas the others were sorely lacking.