I can't name a single "best", just a number of them that I enjoy listening to.
Ann Wilson (Heart)
Floor Jansen (Nightwish)
Tatiana Shmayluk (Jinjer)
Taylor Momsen (Pretty Reckless)
Melani Hess (Dio Knights)
Ann needs no introduction, Floor is simply gold, Tatiana is the black sheep with a wide range of sounds, Taylor is the misguided teen, per say, with that sexy raspy voice, and Melani does an AWESOME rendition of Ronnie James Dio.
There are others... but those come to mind quickly.