What is the best drill bit to use to drill a hole in a chainsaw bar?
I sincerely doubt that you should be custom hacking something onto your chainsaw bar. If this was a legitimate attachment that is provided by the chain saw manufacturer then there should already be mount points defined and instructions provided for installing such option. 

Custom modifications to a tool, especially one that is already as dangerous as a chainsaw, should not be done because of the additional danger and risk to safety that can come into play.