In my case it would be some kind of music reproductor that would allow you to travel in time at the moment when a chose song  was recorded at the studio or was played at a concert,you would be sat in front of your computer and a moment later you would be just at the concert that The Who performed for the very first time My generation or at the record of The Ecstacy of Gold from the soundtrack of the good the bad and the ugly by Ennio Morricone.This would be to me the best selfish wish i ever could ask for.