(...)I'm not talking about superiority in every way, but only raw power and strength. From all the cutscenes and FMVs shown throughout the FF series, the villains ARE stronger than the heroes(...)
Yes, in terms of raw power, each villain is (far) superior to the heroes.
The same way that Sephiroth's raw power surpasses Cloud's and everyone else in the FFVII's world, for that matter. Even if Omega and Minerva can rival Sephiroth's power, they can't surpass his will.
And as seen in ACC, even Sephiroth using only his physical strength, was able to overwhelm Cloud.
But Cloud couldn't let himself die just yet. He couldn't leave a deranged godlike being at loose in his world. Tifa, Denzel, Marlene, Barret, Red XIII, Cid, Vincent, Yuffie, Reeve, everyone... he couldn't let them being killed by Sephiroth.
Cloud didn't wanted another friend's death to burden. Hell, part of the movie had Cloud surpassing the guilt he felt about Aerith's death. Now that he could overcome that guilt, he was going to let that happen again? No.
It was this rage, this desire to protect his loved ones that allowed Cloud's spirit to burn into its max and unleash an attack that defeated Sephiroth.
But i can't see the Omnislash V6 rot away and destroy a planet the way that the Negative Lifestream could.
So, bottom line is that Sephiroth is indeed the strongest being in FFVII, but Cloud, driven by its dreams and desires, was able to overcome his power disavantage and defeat Sephiroth.
At the end of the day, Sephiroth is the strongest and Cloud is the victor.
And that's all, folks!