Anything classical will do for me

and I love listening to and playing it whether it's a bad day, an average one, an extraordinary one, an extraordinarily lousy one, etc, etc, etc ^^

but well, some of the recent ones I've listened to in bad days have been (not all of them actual
songs, though):
-Chopin's 'Harp' Étude Op.25 No.1
-Pachelbel's Fugues on the Magnificat (yes, I definitely agree with you, jmenz, just astoundingly gorgeous, those!

)
-Schubert's 'Die Forelle' D.550
-Chopin's 'Czary' from his 19 Polish songs Op.74
-The lass of Glenshee
-Prokofiev's Piano Concerto No.2, first movement
-Massenet's 'Sous les tilleuls' from his 'Scènes Alsaciennes'
-Schumann's 'Vanitas vanitatum' from his 'Fünf Stücke im Volkston' Op.102, as well as his 'Langsam' from the same work
Best regards,

Musikalischer Wirbelwind