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.
How could you miss Winter Wrap Up when it's the best song in the series? It had the best remixes and fan content, and it's the one that gets stuck in my head every single time I hear it.
Winter Wrap Up definetly helped the explosion of fan made content!