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.
Winter Wrap Up definetly helped the explosion of fan made content!