Buffy the Vampire Slayer has two notable unbeatable, godlike characters. The first being Glory, who was by all accounts unbeatable. She was eventually defeated in the form of her human alter-ego, but as a god the lead cast didn't stand a chance. The other would be the First Evil, which went undefeated through the series' conclusion, because there is no possible way of vanquishing the abstract concept of "evil in existance."
and in terms of FF Final Battles, the series seems to go with the trope that friendship/love/teamwork can vanquish any obstacles etc. This does not exclude the AC/C final fight. Cloud is all but defeated by Sephiroth, but because he has more of a
reason to fight, to protect his loved ones, Cloud came up on top. It's actually pretty consistent with all the counters I've seen between them, Dissidia discluded as I haven't played a version I understand yet
So simply put, while I believe Seph is probably more powerful and skilled than Cloud, my hearts&gay bullshit explanation of his wins is that er... it's all about the love, man