Both of these heroines got an entire arc to themselves and proved their fighting prowess. Leafa actually had the edge for a while thanks to her ability of flight, but Asuna quickly took that away from her. Leafa’s healing abilities would typically give her the edge, but the spells take so long that they may as well not even be there. Asuna is more of an accomplished fighter and her flurry of blows would quickly overwhelm Leafa. We now know who the strongest SAO heroine is! (Until Sinon shows up!) Asuna wins.