Since I like going over things...

(SPOILER)Chaos/Garland: Didn't become powerful until he 'sacrificed himself' to the fiends. In return, became the classic 'omg I'm the epitome if evilz!!1' villain.

The Emperor: Same deal here. Gave his soul up to hell to control Pandaemonium, although he could already 'summon hellspawn' to do his bidding anyway. Reasonably powerful.

Zande, Cloud of Darkness: Zande, another guy who's a pawn. (We'll be seeeing alot of that.) Cloud of Darkness, as was stated, was a force of nature, and the standard 'epitome of darkness' you just have to take out to save the world.

Golbez, Zemus, Zeromus: Golbez was being controlled by Zemus, already had poweful sorcery. (Possibly as a result of being a Lunarian.) Zemus was already a very powerful Lunarian, it did take the power of the Lunar Crystals to seal him away. Even then, he did have a bit of telepathy or something to control so many people. Zeromus is the manifestation of Zemus' hatred, our good old 'epitome of darkness', or hatred in this case.

X-Death, Neo X-Death: X-Death (as I recall) was the spirit of one of the trees in the Mua forest, and I recall he had formidable magic powers. Still, he was overcome by the power of the Void, and was engulfed by it. As for Neo...he's just our Deus ex Machina final boss. (See the last four final bosses.)

Ghastra, Kefka: Ghastra was pretty powerful, but Kefka got a little more boost in power, backstabbing him in the end, and becoming the 'nihilistic godlike' villain we all know and love. Essentially caused the destruction of the planet, or more accurately, it's ruin. Very powerful.

Sephiroth, Jenova: Jenova is the 'calamity of the skies', infecting the Centra and being sealed away, not much else to discuss on it. (Her?) Sephy was a very powerful Jenova-infused SOLDIER, and with great willpower too. (Enough to resist Jenova's control, at the least.) Becomes a nihilistic god, yaddayadda. Summons that spiffy meteor, at least he's got that going. Pretty powerful.

Ultimecia: Pretty powerful. However, can only possess Sorceresses, and couldn't compress time until the gang helped her. (If only to defeat her.) Comes with the standard package, angelic appearance, and big celestial attack. Can't go home without one.

Kuja, Necron: Kuja was powerful, if only after he discovered Trance. (He did blow up Terra, at least.) Necron is also our standard Deus ex Machina final boss, nothing terribly special about him. Well, they did combine the 'epitome of darkness' and 'nihilistic god' into ONE whole final boss, they must be doing something right.

Seymour, Sin/Yu Yevon: Seymour portrays our nihilistic god very well, if only because Spira has some wacky standards on dying...oh well. Yu Yevon is much like Zemus in the way that he's the little puppeteer behind it all. Too bad you have to dig a bit deeper to get some back story on him. (I guess being inside Sin for so long turns you into a weird parasite thing...huh.)

So either they're nihilistic gods who become powerful over time, are some 'force of nature' or 'epitome of darkness', or are already moderately powerful and have been controlling everything and summon/become the epitome of darkness. Works for me.