How do you rate a champions power level?

I'm interested in how other people on the boards (as compared to Riot's way of win rate vs play rate vs ratios for people who play the character a lot) decide on how powerful a character it is.

Is it just whoever is the most frustrating to play against? Is it by raw stats? What's your basis for your choices?

For me it's flexibility paired with their raw damage and the amount of support they are given by current itemization.

So, like Annie is really binary (less flexible), but her raw damage is also really high as an example.