The First Cut Is The Deepest was written by Cat Stevens and was first recorded in 1967 by P.P. Arnold (real name: Patricia Ann Cole), a member of Ike & Tina Turner's Ikettes, for her debut album The First Lady Of Immediate. A version by Keith Hampshire reached #70 on the Hot 100 in May 1973, a version by Rod Stewart reached #21 in April 1977 and a version by Sheryl Crow reached #14 in March 2004.
Here is Cat Stevens' version, from his New Masters album, released December 15, 1967. Big Jim Sullivan (real name: James George Tomkins) is the guitarist.