Good question...
At the end of the day my favorite VA is most likely Kikuko Inoue (best known for her role as Belldandy in Ah! My Goddess), followed closely by Yokoyama Chisa (best known for her role as Shinguji Sakura in Sakura Taisen).
Like a lot of things, this one is tough as there are many that deserve mentioned
in the same breath.