It may seem a bit childish, but my favourite theme song comes from the Spongebob Squarepants movie! It's not about the lyrics, it's the combination of the text and the music that give me motivation and a sense of accomplishment!
This was really a beautiful idea, though and I am going to have my students present their own theme songs and tell me why they chose them!!! Nice way to start a lesson after a long summer vaccation!!!