The Akutan Zero.

Wiki here.

Basically, the Japanese Zero was superior to the American Wildcat, then we captured one intact. We test flew it and found its strengths and weaknesses. The result was the Hellcat with which we designed specifically to kill Zeros and it worked.

There have been several instances where there have been celebrity defections where the pilot has landed the latest ride intact.

What was the response of the receiving country? Are there any instances quite this dramatic?

There was the FW 190 which was superior to the Spitfire. The British got a hold of oneand was able to modify the Spitfire

About the modifications.

Victor Belenko flew his Soviet Mig-25 to Japan.

Among various others.

It should be noted that there are disagreements over how much intelligence was due solely to obtaining the Zero. The F6F was already in design based on reports from the field.