What kind of icing is the best one to use under fondant?
I use any kind of buttercream...peanut butter, vanilla, chocolate. I think a lot of people use ganache I believe. You just have to get the buttercream really smooth I think before putting on the fondant.
I like to crumb coat in buttercream then add ganache. I know people say you don't need to do it but I like the taste better.
You might want to try my "2 of everything icing" that is in the recipe section of this site :)