As you can see my list is based on strong female protagonists, though my definition of 'strong' varies.
-Some of the girls are not only mentally strong,but also physically.
-Some have a very strong will and won't let themselves be stopped by others of doing what they want.
so it depends on the definition.
I think what they all have in common is that they do not match the often used stereotype of female protagonist in shoujo mangas.
Allthough not every manga of my list might fit your taste, I'm sure you find what you're searching for anyway :)