Among the 3, I think Pete has the best serve, especially his 2nd serve, the best ever.

Pete's serve is powerful not only because of the speed,(almost every player can hit up to 120-130mph these days) it is because of the spins on his serves. He accomplished that extra spins from his shoulder turn by a curve ball service toss, you will see his left hand goes up from his right hip (right pants pocket area) and when he makes that curve ball toss. So far he is very unique on the ball toss. Every one else just toss the ball striaght up, including Federer. That unique shoulder turn generate the added spins also enable Pete to hit his serves more accurately.

Federer though added a few MPHs on his serve by doing someting else. He starts his service by having his weight on his front foot, the left foot, and then rock back, and shift his weight to the right foot, and finally shift the weight again forward while his racquet hits the ball, and hence added some of his weight to his serves.

Roddick's serves though is different than both of them, his speed is mostly generated by his huge knee bend, huge back bend, powerful shoulder and arm strenght, but what makes Roddick's serves different and go up to 150mph is his left shoulder. As his racquet hits the ball, his left shoulder actually makes a throw back direction (pulled back to his body)and therefore enable his right shoulder and hand goes forward even more and generate more racquet head speed.

For those of you play the game yourselves, you can try all three of the above techniques. You might find your serves have a few added MPH. Haha... you never know, you might have a Pete Sampras serve in you. Don't under estimate yourself!