Here are my reasons why Rinoa is not Ultimecia:

1) There is resemblance, but they are clearly not the same person (find two photos next to each other that disproves this, not anything with transition effects).

2) Their personalities are completely different. Rinoa is bubbly & warm, happy to live in the moment. Ultimecia is dark & broody & depressed & so scared of losing something she'd rather be alone (sounds more like Squall than anybody else). From a literary perspective, there is no way such a change would occur. Why would they (being Square) build up a character in such a way, when we are supposed to infer that she completely changes?

3) Many people who propose they are the same person suggest that the reason Rinoa became evil was because she lost Squall. So: why does she constantly try to kill him? Both as Edea and herself. Why does she want to be completely alone? Surely she'd want to be with Squall.

4) This is tied into 3. Proponents of this theory also say that Rinoa's line in the Ragnarok, which she says this moment stayed still forever, is linked to Ultimecia saying time doesn't stay still. Same argument as 3. Why choose to compress time so only she could exist if she could rather be with Squall? Surely, if she was so depressed over Squall, she'd rather die than be alone for all eternity.

5) Ultimecia can go back into time and possess different sorceresses. Again, if Rinoa is Ultimecia, why didn't she possess herself to protect Squall.

6) Odine says that Ultimecia lives many generations in the future. Seeing that this would be substantially longer than one's average lifespan, it would be unusual if Rinoa was a) alive & b) very young. And looking different at the same time.

Do I need to continue? You can argue that this theory is possible. & yes, it is possible. It is also possible that every event in the game is a dream that Squall/Quistis/Matron/the sick nurse was having, just as it is possible that we're all actually living in the Matrix. Is it probable/plausible. I don't think so.