Diggy Diggy Hole (written by The Yogscast, and performed by Wind Rose) is, in my opinion, the song that Ragnarock was made for. It's not included in the game, but it is, by far, the most popular custom song.

Wind Rose cover (2019):    • WIND ROSE - Diggy Diggy Hole (Officia...  
Yogscast original version (2014):    • ♪ Diggy Diggy Hole  
Simon's original original version (2011):    • Diggy Diggy Hole