"Best" is relative. Cheapest is probably the shuttle that operates (I think it is called McCarthy Shuttle but am not positive). You can also take the shuttle one way and fly the other.
Depending on where you overnight, you might be able to use Enterprise and be allowed to drive the McCarthy Road. I know I saw that on one of the lodging websites last year. Not sure if it is still allowed.
When I visited McCarthy a couple of years ago I took the risk and drove my rental car there, knowing I was fully responsible for any and all damage, breakdowns, etc. The cars really do get filthy when driving that road, so it took a while to clean the car inside and out before returning it. My guess is that one main reason rental companies don't want their cars driven on such roads is that it takes a long time to clean up them once they are returned.