Complex sentence: His most successful single as a songwriter is his 2003 song "What Hurts the Most," a Number One single on both the Hot Country Songs and Hot Adult Contemporary Tracks charts for Rascal Flatts in 2006, which was also a chart single for Cascada.
Split into simpler sentences: His most successful single as a songwriter is his 2003 song "What Hurts the Most". The song was a Number One single on both the Hot Country Songs and Hot Adult Contemporary Tracks charts for Rascal Flatts in 2006, which was also a chart single for Cascada.