This is a tough one. I Won't See You Tonight PT.1 is so emotional, and honestly hits me hard every time, but Welcome Home by Coheed and Cambria is so much better made. My all time most listened to song is MyBad! by Grioten (I think)
Right now, I don't have a single favorite song, but these are my most listened-to ones at the moment:
1912 Farewell of Slavianka
Cruel Angel's Thesis
Idol
Beethoven Virus
Cossacks in Berlin