AC Sephiroth had control over the negative lifestream, lifestream in the world of FF7 is the power of everything, everything was made up of the lifestream, from materia, to buildings, to all life, it also has the power to destroy the planet, it disinegrated the giant meteor very fast, with a gesture, he covered the largest city instantly.
Sephiroths willpower is so powerful that when he first died, he reformed his body back at the northern crator, the 2nd time he died, he infected the entire planet, creating his own plague and even his own negative lifestream, he has the power of Jenova, which means he can shapeshift, go intangeble, powers at unknown limits, etc.
He has created life, and the beings he created were very powerful.
His TK is powerful enough to hold back Holy, a destructive force, powerful enough to destroy the planet, with a nod, he collapsed half a giant building, he has various summons, like supernova, which is capable of destroying planets.
He has been stated to be most powerful character in the FF7 world, which includes the chaos, weapons, and even the goddess herself.
He was capable of transforming the entire planet into his own ship to soar the cosmos
A little cool spar of Sephiroth(his weakest form) when he was still in SOLDIER, before he gained most of his powers and was insane
YouTube video