I don't know about the best but I do own a hand filer from Speed Pro and a electric one from Pro Form
Both do the job but the electric one is way faster and easier
Be careful on the electric one and when switching from the Moly top rings to the second cast iron ring, the cast iron rings are way softer than the top moly rings and it is real easy to make the gap to big
Take your time on this procedure especially if you haven't done a set already on either type of ring filer
File a little and check it, and and do that again until you get what sizes you want