I think Tyson should be fourth...he didn't just beat fringe contenders in this period, he won two world titles (Bruno and Seldon) by knockout, and also notched up KOs over decent opponents like Botha (and Golota a little later on)....in my opinion, that's a hell of a lot more than Tua,Holmes or Mercer accomplished in this period.
Put Tyson at 4th and move everyone else down a notch, then it looks like a decent assessment.