I think for me, it would be Shake it off- Taylor swift!:)

Ed Sheeran-Kiss Me

https://www.youtube.com/watch?v=kFfKb_WEkCE

"Always look on the birght side of life" by Monty Python 

its so meaningful song for me :)

Jupiter, The Bringer of Jollity, from Gustav Holst's The Planets.

The Largo from Dvořák's Symphony #9, "From the New World."

Probably Courage by Tomatsu Haruka for myself, or L's theme song from Death Note. 

Otakus for the win! 

Courage Song

https://www.youtube.com/watch?v=aR_r-MqEwXo

Courage Lyrics (English)

http://www.lyrical-nonsense.com/lyrics/haruka-tomatsu/courage/#page=English-tv

L's Theme Song

https://www.youtube.com/watch?v=lgjKp5IXz5I