For me, it depends on why I'm in that mood.
But among my most listened to when I'm down are Linkin Park, Breaking Benjamin, Within Temptation, Dave Matthews Band, Pentatonix, Celtic Woman, and Lindsey Steriling.
Other music I just lose myself in while I listen. It's like I'm transported to a whole other world. This is what I do most often when I feel down. Music that does this includes works by John Rutter, Gustav Holst, Frank Ticheli, Maurice Durfle, Beethoven, Rossini, Aaron Copeland, Mendelssohn, hymn arrangements, movie scores, and Celtic music.