In terms of extreme power, one should not forget (in no particular order):
1.Flipper (the most powerful groundstrokes of any era)
2.Berasetegui -the hardest pound for pound? Maybe the hardest period.
4.Bruguera (as much spin and racquet speed off the forehand as Nadal)
5.Korda (the only player I remember who would hit SURE cold winners off the forehand and backhand, because he had racquet speed equal to the most powerful players of all time, great timing, and would hit it flat!)
6.El Aynaoui/Krickstein/Courier - short backswings, economical, built entire game around
9.Larsson - a little like Del Potro, with every bit as much power, though he moved horribly! lol