Joe Posnanski's Best Players in Baseball (from 1970-2009, based on Win Shares, via David) confirmed my childhood belief that Will Clark was the best player in baseball for a couple of years.

My reasons for his best-player status may not be as statistically valid, but they include: Clark homered in his first major league at bat, off of Nolan Ryan; and, as was pointed out in the comments, his 1989 NLCS grand slam against Greg Maddux was, well, thrilling: