If not Zidane, then who? Vivi lacks charisma despite going on his journey of self discovery, Garnet becomes almost useless in Disc 3, Steiner and Freya aren't in a lot of Disc 2, Eiko and Amarant only turn up near the end of Disc 2 and Quina isn't the most inspiring either.

Zidane is in (almost) every single bit of the game and he guides the team throughout the entirety of the game. Hell, without him, they would've gotten stuck in the Ice Cavern. That would suck.