Omnislash is clearly a PIS move, I'm certain it would take out Chaos as well, Sephiroth didn't use any of his powers in that fight except a little TK which brought down the building.
Sephiroth held holy (which can END EVERYTHING) with his TK for months, he has the full power of jenova and power of the lifestream, which is the power of the planet, nothing beats that 😬
"Kitase said that Sephiroth's existence and will is extremely powerful. There is nothing stronger, nothing above him."
"Zack was chosen as the main character as a representative of a BC era SOLDIER. Sephiroth is already the strongest in the world so there's no room for growth, and we couldn't possibly see ourselves making a game over type situation where he loses"
So since Sephiroth has the power of the planet, powerful willpower that is strong enough to hold even Holy, full power of the Jenova entity, stated by the creators that there is NOTHING above Sephiroth, and Sephiroth is the strongest in the world..
I'm sure Sephiroth is the most powerful FF7 being.