i thought there already was a thread on this somewhere, oh well. here are my favorites - alissa white gluz (the agonist), brody dalle(the distillers), tarja turunen (nightwish), angela gossow (arch enemy), morgan lander (kittie), dolores o'riordan (the cranberries)
There aren't a lot of banRAB I like that have females, but I am a huge fan of Sharon den Adel (of the Within Temptation fame), she has such a powerful voice.
