I haven't taken the 30 day opera challenge because I don't tend to think of music as consisting of favorites. However, I know that when I was younger and people would ask me what was my favorite, I would unhesitatingly answer: the "Erbarme dich" from the Bach St. Matthew Passion. It comes at the place in the passion where Peter has denied Jesus 3 times, exactly as Jesus had said he would, and now he pleads for forgiveness.
On YouTube there is much discussion about the tempo in this version by Marian Anderson, but for me it is exactly perfect. The violinist is Joseph Fuchs, and the conductor is Robert Shaw.
I once did a harmonic analysis of it, and it's incredible. I have always loved this piece beyond loving. Overlook the flaws of the recording and allow it to sink into your soul.