"Ultimecia is the most powerful villain in FF ever, because she's absorbing all time and existence."

Yeah well, if we cast plot-protection on the villains and have em battle her one on one, she would still get beaten by numerous villains.

"Ultimecia is the most powerful villain in FF ever, because she's absorbing all time and existence."

But, being the most powerful would mean being able to defeat and kill all villains. Like I said, if we cast plot protection on other villains, Ultimecia wouldn't always win. She lacks the power to do so.

"Ultimecia is the most powerful villain in FF ever, because she's absorbing all time and existence."

But...

"Ultimecia is the most powerful villain in FF ever, because she's absorbing all time and existence."

There, I summed up the entire discussion with The Crystal in one post. There, Ultimecia is the most powerful ever, because she's absorbing all time and existence. If the villains would be immune to her absorbing, she would still be the most powerful villain because she absorbs time and existence and thus would kill all those other villains with ease without using her cheap 'let's absorb!'. Of course, this won't happen at all, but she's still the most powerful because she's absoring all time and existance.