I thought it was important to clarify something:

"Power of plot" is not a quantifiable type of power, but a "qualitative" type of power. Although we're doing a measurement of Sephy's power (and the power of all other villains) in a qualitative method, "power of plot" is not something you define by words such as "little" or "much", nor with words such as "weak" or "powerful".

Power of plot is, essentially, to have the right power when plot dictates it. If we were to measure power of plot based on use, and compare all people who have power of plot against each other, Sephy wouldn't be even 0.000001% as powerful as the king of power of plot, Silver Age Superman. With a family, defined set of powers, being pretty much undefeatable and having super-everything (even super-ventriloquism and super-hypnosis, that's how he hid his secret identity), he was the definition of unbeatable. Of course, his power of plot ran out with Crisis on Infinite Earths.

Oh wait, we have Batman as the absolute ruler of power of plot, and he's for all concepts human. When you have "Bat Shark-Repellant Spray" on the utility belt, you know he's crazy prepared. To ridiculous extents, that is.

Evidently, power of plot works on a meta-scale, in which stuff is essentially added to patch parts of the story which might be shaky when a character has an undefined set of powers. If your powers are, say, much like the sidekicks on Sky High, you really can't do much with them; now, compare that to the power of the Scarlet Witch (yet another character with Power of Plot running on steroids), and you can pull off entire storylines with it (as in, House of M and subsequent "no more mutants" stories).

One of the traits is that such characters will be considered, plot-wise, insanely powerful (because you really can't define the limits of their power). Villains usually have a little bit of "power of plot" to fulfill their plans, usually serving some purposes: surviving a sound defeat of the heroes (or just being plain invincible on one battle), appearing when least expected to take the McGuffin (the artifact which everyone seems to look for) out of the heroes' hands, pulling off incredible stuff such as suddenly convincing an entire kingdom the heroes are actually the bad guys, and so forth. Some uses of "power of plot" aren't actual shows of power in first place; instead, they are resources used by the storyline writer to advance the plot. Some are perfectly explainable (such as Kefka's insanity and descent into evil, Sephy's descent into evil and so on) and some just don't seem to do so, which might lead into plot-holes (the realm of discontinuity for Superman before CoIE).

As Heartless mentioned, villains have essentially undefined amounts of power of plot until they are defeated by the heroes (whom also have power of plot abilities, but not the same as the villains). In the case of Sephy, though, it seems power of plot didn't really ran out, unlike in the case of others whom got what they deserved; what happened was that plot needed him again and did the revival Aeris actually deserved (but didn't because that would otherwise ruin the plot, no?).

Power of plot, as stated, is no true measurement of the individual's perceived power potential. Power of plot is not a bad thing, either. However, a creator can use power of plot in a bad way, which leads to discomfort between people.

Odin (and only Odin, I don't want to extend this a bit further), consider the following: say we're dealing with FFVI. We all know Kefka was soundly beaten, his will eradicated, the power of the Espers dissipates, Terra becomes fully human instead of half-Esper (never mind the genetic troubles with that), and so on. Then, assume several years later, there's a video sequel to Final Fantasy where, somehow, Magic has revived, people find Daryl's body and reincarnate Kefka in that body; furthermore, the power of the Espers that was supposed to wane suddenly starts reappearing, Terra's powers reawaken, and Terra (still addled about why her power has reappeared, with the consequence that it might never have left at all) has to face Kefka, with support from all the characters of the game (even Mog, Umaro and Gogo). Let's also define that the Warring Triad is the most powerful thing on the world of Final Fantasy VI, that Sakaguchi himself stated that, and that Kefka is the incarnation of the Warring Triad, thus he's the most powerful individual in that world by definition. Now, let's assume Final Fantasy VII was made, and it ended exactly as it ended before. However, Crisis Core, Advent Children and other sequels never existed.

Finally, let's assume the same situation stands, with most people (I would claim myself included) defending Kefka, while others defend Sephy.

One: which might be the most powerful then? Second: did Kefka, in this exercise, was the most powerful because of power of plot, or because the story has defined him as the most powerful? Again, I only want Odin to answer this; no analyzing, no trying to aid; let him answer, and I'll answer later, and then you might comment.

If you wanna comment, Xanatos' point is a good one, though I consider the story is best told entirely, even if that would otherwise damage the story (though I also find it's invaluable to fix the damage done to the story). Case in point: Star Wars. Love it or hate it, Expanded Universe material is canon, and it expands on both what happened before AND what will happen in the future, and it makes some characters far more epic than they would have otherwise been (Obi-Wan, Palpatine), even though it might have done some damage to others (Darth Vader). There are some things that definitely should have gone out (Jar-Jar!!!!), but otherwise the plot behind the first trilogy wasn't so bad. I mean, if you saw the ending of Attack of the Clones you might have been as exited as pretty much everyone about what happened; also, it truly cemented Palpatine as an utter bastard of a villain. But some people consider only the first trilogy is canon, much like most people might wanna forget Kirk's trend on Star Trek and focus mostly on Picard (or viceversa). Still: if what Ultimania says has some validity, then the only reason why Sephy has immortality is caprice of the creator? Furthermore, is that a revision done by Kitase or was it originally on Sakaguchi's plans when pitching the original concept of FFVII (considering he also dealt with the concept of immortals in Lost Odyssey, which may be further proof or a massive "take that!" to FFVII's deal with immortality).