P-51 was a good plane, but as many others it had its advantages and disadvantages. P-51s were used in high numbers and usually had an advantage of 10 to 1. They had high range (the main mission - escort bombers, that no other fighters could do) and were a little faster than German planes like Bf-109, not jets. But any veteran or historian will tell you which plane you want to be in when fighting against Bf-109, P-51 Mustang or Spitfire, they'll choose Spitfire. Bf-109 could out turn and out climb P-51, if Bf-109 had a good pilot. I think Bf-109 and Spitfire were rivals throughout the war.
A lot of experienced German fighter pilots died on the Russian front, and it helped the US air force. The most widely produced Russian fighter was Yak-9, it wasn't the best, but there were many of them.