It is in the classical section, so much as there are many others that I find calming, I'll limit mine to classical.
Hallelujah Chorus from Handel's Messiah. Can be a little 'overblown', but it's soul stirring and a piece I never feel worse for having listened to. Trite? Predictable? So what. It is a stunning work and if any music can lift your mood, this should.
Elgar Violin Concerto. The Menuhin is well known but the Itzak Perlman I find more moving. There is so much emotion in it that whatever you are feeling is totally wiped. Despite my preferences and general collection, I would probably choose this as the one piece I would take with me.
Finlandia. Again, an obvious choice and overly commercialised, but nevertheless, a stunning work that always makes me feel better mentally when I play it.
Grieg, Wedding Day at Troldhaugen. You cannot listen to this without mixed feelings, but it is such a perfect composition that it always lightens my mood.
Messiaen. Transports de joie. Organ work. Needs a system capable of handling the really low organ notes. And with enough belt to really hit it when the crescendo comes in. Magnificent, challenging and stunning. Takes you into a different universe.
Many more.
This is what music does. It takes you away from the shit and puts you in a better place.