I recommend Len Deighton's fine novel "Goodbye Mickey Mouse", about a P-51 squadron flying bomber escort in the later stages of WWII.As to the best WWII fighter, wasn't it the top general of the German air-force who when he saw that the P51 aircrafts were able to stay with our bombers for the entire flight because of improvements, including drop tanks to extend the range, said something about the war was lost.
In the book, the pilots lament over having to fly P-51 Mustangs instead of the more durable P-47 Thunderbolts. Someone reminds them that "Uncle Sam pays $100K for a Thunderbolt, and only $50K for a Mustang".
I can't say this is entirely accurate, but tend to believe it as Deighton is generally an excellent researcher.
Agreed that the appearance of the drop tank in late 1943 allowed the American fighters to accompany the daylight bombers all the way to the target and back, changing the air war in Europe dramatically.
In actuality, the outcome of the war in Europe was pretty much determined by the end of 1941; the proposed invasion of England had been postponed indefinitely the year before, Hitler had unwisely invaded the Soviet Union and the Wehrmacht was now bogging down in the cold winter, the tide was beginning to turn in North Africa and in the North Atlantic, and the US had just entered the war. But I can see how the increased daylight bombing would have brought that home to the German people.