I think its safe to say that all the characters are fairly useful in FFIX (One of the many reasons why i love the game so much) But it would be a fools gambit to say that there is any character greater then Vivi (The best reason to love the game)

Why just look at these factual (Oh its a word alright) statistics shall we?

1. He's a blackmage...doesnt get much cooler than that
2. He uses black mage
3. Black magic trumps all

Also just for the record my party at the end of the game was Zidane,vivi,garnet, and freya and it was freya's trance that finished off ol necron for me so she holds a dear place in my heart (i always had a good feeling about her) She never did much for me but when she did it was always well worth that filled up space in the ol 4 man party roster...so i say keep the ol gal around...she'll suprise ya (That and everyone else's advice with the whole dragon crest thing is pretty cool too...but vivi is still the king)