And I also believe it to be Rinoa.
This confirms my Rinoa=Ellone theory.
In Final Fantasy VII, before Jenova is removed from the Mount Nibel Reactor, she is stored in a device that has an angel's face and wings on it. This confirms that Jenova is Ultimecia.
This theory is something I came up with a few years ago, in response to the many other nonsense theories. Let's look at the evidence again:

* Jenova can travel between worlds. FFVII and VIII take place on different worlds

* Jenova and Ultimecia both have phenomenal magical power

* Both appear to be female

* Both have long silver hair

* Both have inhuman eyes, sometimes with a red glow

* Jenova can change her appearance. Since Jenova and Ultimecia look differerent, this supports the view that they are the same entity using a slightly different form

* Both crave absolute power

* Jenova fought the Cetra by appearing in different forms, to deceive them and influence them. Ultimecia took over Edea, using Edea's form to deceive and influence Galbadia. Similar methodology

* Ultimecia took Seifer as a knight, empowering him to defend her. Jenova's power exists in Sephiroth, the greatest warrior ('knight') of FFVII. Both male minions are devoted to fulfilling their mistress' wishes. Again, the methods are very similar

There we have it, folks - by assessing evidence from each game, it can be concluded that Jenova and Ultimecia are, in fact, the same person. The only remaining issue is which came first, chronologically