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.
This Day Aria is definitely what got me paying attention to the music in the shiw. My absolute favorite still might be The Art of the Dress though. Early season music just hits different