Female Go champions?
Rui Naiwei has won the Guksu (the 43rd open Guksu title in South Korea, 1999) and the Maxim Cup (2004).
Wiki: https://en.wikipedia.org/wiki/Rui_Naiwei
As for comparitive rankings, this article from 2016 may be of interest to you.

AlphaGo’s win makes it the only non-male player in the top 75 world rankings. Yu, the first woman, is number 79, as of Friday.

It also includes this insight:

Most matches are divided by gender, so women at the top of the game don’t get to play at a level that seriously stretches their abilities.