This is a very easy choice for me :
Sumeragi Kururu from Air Gear by far, more than any other anime girl out there.
Kururu is the most useful and supportive female character in any manga / anime I have ever read. She helps the main character Ikki grow so much throughout the manga, she goes so far out of her way to help him get stronger, she is primarily responsible for his power. I really want to punch Ogure Ito in the face for writing such a good female character, and not making her the main heroine. Screw the childhood friend trope!