HarryWild, good call on Philippousssis! Having seen him play more than a few times up close in match and in practice, I can say that it was the only time (other than Del Potro in US Open semis against Rafa in 2009) I felt like someone was going to explode the ball every time he struck a forehand - I remember laughing at myself for thinking back in the 80's that I would never see a bigger forehand than Lendl's, and then again in the early 90s when I thought the same thing with Courier and Agassi.
Also, I am surprised that Robin Soderling has not been mentioned (I wonder at this point if we'll ever see him play again). The guy's groundstrokes are very heavy on the opponent's arms when he is on a roll, especially when he strikes those cannon shots on high balls about his shoulder level.
Having said all that, Rafa's forehand when he is swinging his arm full-force is a visual phenomenon.