This song just had to be included on the list! First sung in the pilot episode, Dont Stop Believing brought fame to Glee and therefore Cory. The song is a group number, sung by the first five members of the glee club; Finn, Rachel, Mercedes, Tina and Artie. In the episode, Will Schuester had given up the glee club and was soon to change careers to accountancy, in order to support his unborn child. But then he heard the glee club practicing this song and it instantly changed his mind: he had to go to nationals with the group. The song was repeated a couple of times throughout the run of the show and has been performed live by the group. However, it is the original version that I believe is the best.