The answer actually varies by the type of herb you are using. The rule that applies to most situations is that you would use Everclear for fresh herbs and either 100 proof or 80 proof vodka for dry herbs. The main thing that would contradictthe general rules would be that dry resinous herbs like chaparral are best done with Everclear.
Two great books that give better information are:
The Home Herbal Medicine Maker's Handbook, James Green
Making Plant Medicines, Richo Cech