Oh, there are just too many to choose from.
My favorites would have to be Guncakes by the Living Tombstone ft. YellingAtCats, Rainbow Factory by the WoodenToaster, Mirai Start! (Euro-Senpai Mix) by Eurobeat Brony, The Massive Smile Project by Various Artists, and my all time favorite would have to be a classic, Beyond Her Tomb by TwentyTen (Consisting of MicTheMicrophone, iBringDaLULZ, The Living Tombstone, and Glaze/Wooden Toaster)
Well sorry for rambling there, But those are my favorite pony fan songs.