My immediate thought when I saw today's prompt (What song did you listen to in 2011 to completely change your mood? Think about ways you can you add more music to your life in 2012.)?
Cee Lo Green's "F*ck You." And yes, the unedited version. "Eff You" just doesn't have the same effect when you're pissed off.
In all seriousness though, that song and possibly "Raise Your Glass" by Pink are the two songs that stand out in my mind from this past year as being what I turned to when I was in a pissed off mood and just needed to let it out. And they made me feel better, every time.
When I'm in a happy mood and want to stay that way? Iron and Wine or Mumford and Sons. What can I say, I don't have particularly eclectic tastes and I tend to stick with what I like (which may or may not drive my husband crazy).