The best caliber for Mulies is the gun you are the most comfortable with and shoot the best!
Not being short with you but far too many hunters spend way to much time and money on this question.
Anything from a .243 to 338 mag will do the job but a rifle chambered in 270, 308, 30-06 will take down any North American game successfully with a properly place shot!
The questionable piece is the shooters capability to consistently put the round in the vitals. If you have a rifle you trust, shoot well and is a legal caliber then use that rifle!