Considering that you haven't read theories by the vast majority of the people who have posted in this thread, how can you possibly make that assertion?
I have read many theories and I just want to make better theory.

Ultimecia is Quistis because they both have blonde hair, they both like to dominate others and they both fail at their job:

Ultimecia fails as an evil sorceress

Quistis fails as an instructor
We are talking about TC and TD in this thread,so please stay on topic