Cardboard Gods thinks it might be Reggie Smith. He was my favorite Red Sox after Yaz! I was not happy at all when he got dealt to the Cardinals.
1. He was an outstanding hitter. He hit for power and average and drew a lot of walks, posting lifetime batting/on-base/slugging averages of .287/.366/.489, strong numbers that are even stronger when you consider that he spent the prime of his career in pitcher’s ballparks, and all of his career during a pitcher-friendly era. His lifetime OPS+ was 137, better than the career marks not only of Singleton, Simmons, and Murcer, but also of Johnny Bench, Joe Morgan, Tony Perez, and Carl Yastrzemski, to name four Hall of Famers from Reggie Smith's era.
2. He was an outstanding fielder. Murcer also gets points in this area, and though Simmons wasn’t considered a strong defensive catcher he rates special mention for manning that demanding and vital defensive position while still anchoring his team’s offense. The slow-footed Singleton, on the other hand, could not compare to Smith as a fielder. Smith was a Gold Glove-winning centerfielder early in his career, and was an excellent rightfielder throughout his prime. Singleton is very close to Smith as a hitter, and you could argue that he has an edge in offensive contribution to his team simply because he was able to appear in more of his team’s games than Smith, who often struggled with injuries. But Smith’s fielding, in my mind, at least brings him even with Singleton as a player.