I was kidding about the "Quistis=Ultimecia" thing.
I believe Ultimecia is Ultimecia, not anyone else. A sorceress born in the future, who inherited powers - and possibly a couple of memories - from a sorceress who got them from a sorceress, and so on back to the sorceress who got the powers from Rinoa.

Pick any character at random, and you can make a theory about how they're 'actually' Ultimecia; however, any such theory depends on two things: (1) that all sorceresses can alter their appearace (Adel is the only one who is known to do that in the game), and (2) that sorceresses do not age. Edea's youthful beauty does not prove this; all Square characters have smooth skin and shiny hair, even Grand Maester Mika in FFX.

Theory #211: Fujin=Ultimecia! They both have silver hair, and a similar stature. You see, Fujin secretly admired Squall; having her feelings crushed by Rinoa drove her into madness, so she found a sorceress and took her powers, eventually becoming Ultimecia. Remember the parade in Deling City? Ultimecia, using Edea, tried to kill Rinoa. This proves beyong any doubt that Rinoa cannot be Ultimecia, and that Fujin actually is.

Theory #212: Mog=Ultimecia! Moogles are second-class citizens in the FFVIII world; we know this because MiniMog (from the PocketStation game) and Ms Moogle are the only moogles mentioned in the whole game. Ms Moogle owns a coffee shop that Selphie liked. A menial job, serving the selfish desires of humans. Clearly, Ultimecia is a moogle who wanted revenge on humanity. What better revenge than compressing time to become a Goddess and absorbing all creation?