I think there has to be a certain amount of "Horses for courses", at the time they were needed, in the equation. The RAF did not need much bomb carrying capability during up to the Battle of Britain and so the Spitfire and Hurricane were what they needed at the time, ( apart from the fact that is what they had that worked at the time).
The Mustang excelled in long range escort and was more a fighter than a bomber. Probably did a lot to help the effectiveness of the daylight bombing effort.
No doubt the Thunderbolt was lot more effective against ground targets, its carrying capacity was probably one of the design aims. Seems to have been a great all rounder though.
I think it is hard to select a "Best" when the designs were probably done with a different purpose at different times.