What is the best potato to use for a rosin baked potato?

From July to September you are most likely to only be able to get the Russet Burbank variety. This is what the Chain Cracker Barrel used to serve year round beginning with the harvest in September and October. In September to about June the Russet Norkotah from Idaho is another choice. 

Check with your local health department before starting up the Rosin baked potato. Many do not allow it anymore.