Bastila Shan and
Meetra Surik (The Jedi Exile) are defiantly most powerful Old Republic era female Force users. Those two were both lovers of Revan, and helped him remain a Jedi in his heart.
My personal vote would go to
Jaina Solo-Fel, but that's more because I know her full story and she will remain my favorite EU character no matter what the new Disney movies do to the Solo kids.
If you go by force of personality, then Leia, Mara Jade, and Jaina all share the top spot. The way all 3 were written in the EU made them all every strong social forces to deal with. I mean Leia had to put up with Han Solo for over 40 years now, could you handle that? lol