This is a silly question isn't it? It depends entirely on your rocket's TWR and the destination. First of all, the most efficient parking orbit, is to not have one at all. Just burn straight into the transfer from the launchpad. But of course, it is much easier to use a parking orbit. The next best thing is to transfer from a low altitude of 69,078 meters. Then again, you can't have a TWR of infinity, and it is more efficient to burn along the maneuver node instead of the prograde. You'll have to set a parking orbit altitude higher based on the time it will take you to do the maneuver to prevent your rocket from going into the atmosphere again. You can improve your TWR efficiency and prevent dropping into the atmosphere by doing several orbits on the transfer burn too.