For the sake of this discussion, lets say its not Mays who's the greatest of all time (but understand that Mays is probably the greatest fielder of all time at his position).
It's got to be either Ruth or Bonds. Both come with hugh asterisks (Bonds = steroids; Ruth = greatly diluted talent pool), however their domination of the leagues they played in was so pronounced that it's hard to ignore them. Ruth never played against a black or dark-skinned hispanic player, however neither did anyone else in the league at his time, and nobody else was close to him when he played. And Bonds almost certainly used steroids, however it was rampant (make no mistake about it) in his time, and again, nobody even TOUCHED him when he was in the league.
It depends whether you consider Bonds' defense to be more valuable than Ruth's pitching, and whose asterisk you are most willing to overlook.
I lean towards Bonds. One of the best defensive left fielders of all time, a phenomenal base runner, and one of the three best hitters ever (Ruth, Williams, Bonds).
Top-5 position players is probably this for me:
Bonds
Ruth
Mays
Williams
Mantle