There are a few songs that manage to elevate my mood whenever I hear them, but one in particular I'll mention here.
Regardless of what my emotional baseline is -be it content, glum, happy, or anxious- when I listen to David Crowder Band's song O Praise Him (All This for a King), I am instantaneously boosted to a (significantly) better mood. The melody is infectious, and the lyrics are stirring; the song literally gets me every time.
Tonight I was out exercising, and the song popped up in a playlist I'd created a few days ago. Although I was already in good spirits, listening to the song lifted me higher. So, for those interested, I'm including a link to YouTube where you can sample the song: http://www.youtube.com/watch?v=HK3Q_q6as7A
I encourage you to take the challenge: play this song at any time and enjoy insta-elevation. For full effect, try it out on different occasions, as you're experiencing various emotions, and feel the boost.