Zidane was created via a mix of magic and genetic engineering to surpass a character capable of destroying an entire planet. Through his Trance, Zidane's potential could rival those of Kefka, Zeromus, or Lavos (different series, I know).
What separates Zidane from his peers in other Final Fantasy games, is that at the finale, he has not yet reached his full potential. At the end of every other FF game, the main characters are super-charged and as powerful as they will ever be. Zidane is still probably a decade or two away from reaching that.