But Cloud defeated Sephiroth, so doesn't really matter. His Omnislash is teh Imba!1!11

Agreed. Cloud hasn't been on the "upper tiers" of power for a while now. Even in FFVII, there were quite a few entities that were stronger than him.

Characters/Entities in VII: Sephiroth, Jenova, the WEAPONS (All five).

Characters/Entities in the Compilation: Seph, Jenova, the WEAPONS (All eight), Zirconiade, Weiss, Nero, and possibly Genesis and Angeal.

And then there are a few that could probably match him, such as Kadaj, Zack, or Rosso.

I don't belive all Weapons are more powerful than him. I mean, he defeated Bahamuth Sin/Shin alone(his friends only helped him to reach the beast, but he destroyed it with his own Limit Break/power).

And Cloud was in the same level of strenght and speed of a SOLDIER in the begin of FFVII. He and his friends defeated humans, monsters, robots, Turks, SOLDIER 3th 2nd and 1st class, Weapons, Jenova, Sephiroth(4 times) and many other enemies. Weapons, Jenova, Sephiroth > any enemie that Zack faced. And let's not forget that Cloud fell in the Lifestream in Meedel, and probably became a little more powerful than before.
In the end of FFVII Cloud is a 1st Class SOLDIER or above it, and SE said(in the Reunion Files i belive) that he was even more powerful in AC than in FFVII. And because of his adventure to save the world(two times if we count AC) he is more experienced than any 1st Class SOLDIER.

Cloud > Base Weiss, Angeal, Base Genesis(in CC), Zack, Rosso, and Kadaj(Cloud defeated him in the end of AC, you dodn't remember?).

And Chaos Vincent is more powerful than Cloud. Chaos is Vincent's most powerful Limit Break after all. But if we compare Cloud and Vincent's Limit Breaks, Omnislash Version 5 > Chaos.
But this doesn't mater, because Vincent don't have Chaos anymore.

How do you know this ?!