Roger Federer's serve is one of the most underrated serves in the game.
In fact, Federer is a great model for most players at most levels, much better in many respects than either Sampras or Roddick. By studying his motion you can identify all the basic elements that go into building a high-level technical service motion, a motion that allows players to get the most out of their ability on a consistent basis.
Federer's motion is so smooth and effortless that it doesn't attract the same attention as Pete or Andy. What makes Federer less distinctive is probably what also makes him a better model. It's a simpler, more classical motion with fewer extreme elements.
Roger's serve may not have the m.p.h. or be as dynamic looking as Roddick's. It may not have as much topspin or be as unplayable as Sampras'. But it would be hard to claim it isn't effective, and seamlessly integrated into his all-court game. If you were to pick one service motion from the top players to use as a template for building or improving your serve, my opinion is that it should be Roger's.
Grip: A Mild Eastern Backhand
Like all pro players, Federer uses some version of a backhand grip. You could call it a continental, or a mild eastern backhand. If we look at the position of his hand on the racket bevels, most of the palm of his hand is on the top bevel, or bevel one. His index knuckle appears to be in the center of bevel two. This grip works well for high level players, pro players, college players, ranked juniors, even some advanced club players. But you can develop the same basic swing elements with a slightly less extreme grip as well. You could call that less extreme grip a mild continental. The heel pad slides somewhat more to the right and is positioned somewhat less on bevel one and somewhat more on bevel two. The index knuckle slides somewhat to the right as well, to the edge between bevel one and bevel two. This is an easier grip for lower level junior players and many adults. It worked pretty well at the pro level for John McEnroe as well.
Federer starts with his weight forward on his front foot, standing up on his back toes. Then he rocks back and stands up as he starts his wind-up. That initial leaning position is not the key to understanding his stance. That a personal ritual, similar to the way Sampras started slightly hunched over, with his front toes up in the air. The real key is to see what happens as he starts his wind-up and stands up so that both feet are flat on the court.
The key to understanding Roger's stance is when both feet are on the court.
What we see at that point is that his front foot is at a slight angle to the baseline, a little less than parallel. This angle is slightly more open in the deuce court compared to the ad. The rear foot is offset to the left of the front foot, with the toe of the back foot roughly in line with the heel of the front foot. The rear foot appears to be just about exactly parallel to the baseline in both courts. But in the ad court it's also set a few inches further back to Roger's left.