"What Hurts the Most" is a song written by American songwriter Jeffrey Steele and English songwriter Steve Robson. Initially recorded by country music artist Mark Wills in 2003 on his album And the Crowd Goes Wild, it was covered by Bellefire a year later. However, the first version to be released as a single was by pop singer Jo O'Meara in 2005, from the album Relentless. Later that year, country band Rascal Flatts covered the song as well, releasing it as the first single from the 2006 album Me and My Gang, topping the U.S country and adult contemporary charts with it. German band Cascada later had international chart success with the song in 2007. It was also covered by Eden in 2008.

Jeffrey Steele co-wrote the song with Steve Robson, with whom he also co-wrote Rascal Flatts' 2002 single "These Days". Robson presented Steele with an unfinished track, and Steele decided to come up with lyrics to finish the track. Originally, he had wanted to write a song about the loss of his father, but instead went with a more universal theme of lost love. After singing the lyrics, he decided that he liked how sad the song sounded, when Robson suggested that it be recorded in a higher key to sound more emotional.

Mark Wills was the first artist to record the song, doing so on his 2003 album And the Crowd Goes Wild. Jo O'Meara, a pop singer, released the song in 2005 and had chart success with it in the United Kingdom. Faith Hill had also intended to include the song on her 2005 album Fireflies, and although she had recorded the song, her version did not make the final cut.Rascal Flatts then recorded the song as well, and per producer Dann Huff's suggestion, it made their album Me and My Gang. Wills' rendition of the song, though never a single, peaked at number 51 on the U.S. Hot Digital Songs charts in 2006 in the wake of Rascal Flatts' success with it.

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