If you are talking about a jet-powered aircraft than I believe it was Germany's Messerschmitt Me 262. Germany built their first jet-engine in 1936 and recognized the potential for it immediately. A jet-powered aircraft was being developed from the early beginning of the war but didn't become operational until 1944. It saw its first combat in late July, 1944.
Plane for plane, the Me 262 was a game-changer but due to disparities in industrial capacities could not be produced in numbers to alter the outcome of the war. After the war it was reviewed by the allies and deemed to be well-ahead of allied jet designs.
Britain's Gloster Meteor was close behind. Britain's Frank Whittle had actually conceived of the turbojet ahead of Germany's Hans von Ohain (he patented it in 1930), but he received no state support for his project until the war. The Meteor first flew around the same time as the Me 262, but it didn't see combat until August of 1944.