All who have posted have expressed good reasons why their choice is the one most important to the creation of the car we love. So...
Another way of looking at it is, which one could have not been involved and by their absence, would have killed the car from ever being developed?
But even with that you have to be reasonable as so many factors came together at the right time to make the end result happen.
We could say, "what if CS's father never met his wife...?"
AC had already installed Ford engines (6 cyl) in the ACE as well as selling several rolling chassis to the builder of the limited run of CORVACE cars that used Corvette V8 engines and were sold in the US. (The fact that CS raced against one in '59 has been discussed in other threads) So, if CS had not been able to get Ford to front the money to buy the rolling chassis cars he would then get Miles to develop into a race car, would he have given up? Not likely in my mind, CS is a salesman and a very good one, he would have gone to Dodge and then to anyone else to build "his" car.
Just imagine the Cobra with a Renault engine!

Ok, probably not, but a hemi would have been fun.
If CS had been unable to find anyone with deep pockets and a suitable V8 willing to invest in him and he had to give up on the plan, would someone else have had the same idea? What if the builder of the CORVACE had hired Ken Miles to help with developing and driving the car? So many what if's...
AC ACE's were competing in the US with a variety of engines and doing very well. I believe it was just a very fortuitous situation that made it all come together, but it is the beauty of the ACE shape that AC modified to fit the bigger and bigger tires that became available that made the car the memorable vehicle it is. The race success it achieved is due to Miles and Remington development and so many great drivers.
So, who do I vote for as the most important?
All of the above were important to the final outcome but I do believe that another person could have stepped into CS shoes and achieved the same results. He was the catalyst, but not indispensable. So I vote for Miles as the most important role
