is Sephiroth the best bad guy in the series?

[The Crystal]

You overestimate Sephiroth's power. The only thing that really showed that Sephiroth had pretty good power was the fact he could surpress Holy.

He could controll Jenova, was absorbing part of the Lifestream in the Crater, AND in AC, he became a God.

First off when in AC is he ever called a god? Secondly Controling Jenova wasn't much power, that just gave him someone to do his bidding when he was stuck in the northern crater.

You said that holding Holly was an example of Sephiroth's power. But you have to remember that Sephiroth's power is his will. He was holding Holly with the power of his will, and he was controlling Jenova with the power of his will too. Controlling Jenova is an example of Sephiroth's power.

He is never called a god in AC, but if i remember well, he is called a god in the Reunion Files.
In FFVII, Sephiroth wanted to have controll over all the Lifestream, becoming a god. In AC he has his own version of Lifestream that he could completelly controll. And his tainted Lifestream was more powerfull than the pure Lifestream, because he could corrupt the pure one, transforming it in his own version of Lifestream.
The Reunion Files say too, that in AC, Sephiroth is in his most powerfull form ever, and that he is the most powerfull character of the FFVII series.
In AC, Sephiroth is a god.