What was the best plane is very objective and depends on theater and the situation of the user.The P-39 was a very poor fighter in the west because its Allison engine was not designed for high altitude combat a common occurrence in the West but not something that American designers where focused on in the late 30's very early 40's.
In the East however combat was more rare above about 15,000ft this altitude and below suited the P-39 well and was not as well suited by The BF-109 and at this level the two planes where much more closely matched and it then boiled down to pilot and unit skill to highly important but for some reason often over looked factors of air to air combat.The unit that has the better overall pilot skill and coordination is gong to be the winner most times.By the time the P-39 was becoming common in the USSR(mid 1943) the Luftwaffe was sending many units to the West and leaving Easter units with less to fight with.
The same can be said of the P-51 by the time the B model was showing up on the scene the Luftwaffe had already been suffering some losses to P-47s and which got replaced by the P-51B or later in most wings except the 56th which flew various P-47 versions for the duration.The P-51B up was really bad news for Germans not because it was the best from a maneuverability stand point it was bad news because now the Americans where able to engage and destroy Luftwaffe fighters anywhere they so desired and the P-51s where causing an attrition rate on skilled pilots that the Germans could ill afford.While fresh and much better trained compared to their German counterparts for the same time in service where showing up from the US every single week and the Luftwaffe could do nothing to stop this.In my opinion one factor that killed the Luftwaffe was it lack of pilot rotation many skilled pilots died and never passed on their skills to new pilots while many US,British and Soviet where being out to much better use some of the time by allowing them to train others.
I read an interview of a former Tuskegee and he explained the reason that the Tuskegee units performed so well was because they went though much harsher training than others did.They could kicked out for slightest infraction and in some respects they tried to make them more likely to fail than others(because some had the agenda for them to fail no matter what) the result was that they where pretty damn good pilots(if they passed they really and truly passed) and they also wanted to perform well in order to prove themselves to doubters.He said that one guy got kicked out of the program because he did not respond immediately in the proper manner to a question so they dropped him.Yet a few weeks later they hired the same man as a civilian instructor pilot and he trained other men.
