Anyway i'm not going to list any of the old wizards like the founders etc. cause we really don't know anything about them.
1. Voldemort - Dumbledore knows he can't defeat him
2. Dumbledore - He took out 4 powerful wizards in about a second
3. James Potter/Sirius - probobly equals
4. Snape
5. Bellatrix - got lucky with sirius, but probobly the most powerful death eater.
6. Moody - Legendary Auror
7. Dumbledore's Brother - If he's related to dumbledore he must be powerful.
8. McGonagal - old but still very powerful.
After these, the rest of the death eaters, Auros/Order members are of about the same ability. Harry has the potentila to be as powerful as Voldemort but has not nearly reached it yet. This is shown by the way Snape is able to toy with him.