I'll start.
In my opinion, the best song has to be This Day Aria, I mean wow, I never expected them to write such a great villain song that could rival villain songs from Disney movies.
Another song I find myself putting on repeat is Light of Your Cutie Mark, mostly because of the great harmonies of the CMC starting at 1:11, I get goosebumps every time.
Ever After
At The Gala
I think there's a few early songs that pay homage to musicals.
a full version of the musical if you're interested
Edit, another obvious musical one is the Flim Flam Brothers Cider machine song
https://youtu.be/LI_Oe-jtgdI
https://youtu.be/a_8-diklXw4