Some good serves to copy, with what to look at:
Sampras - Technically copy the very controlled consistent motion, with plenty of back and leg power. Tactically copy the way he keeps his cool on the first and second serves - both always are hit with decent power (not excessive amounts), a lot of kick, and good placement.
Nadal - Technically you want to copy the way he gets exceptional consistency with a hard to attack shot by focusing on spin instead of power. Tactically you want to copy his high first serve percent, the way he moves the opponent around the box and the way he keeps them guessing with spin.
Roddick - Technically you want to copy the very simple takeback and toss, which prevents errors, and the huge coiling he achieved with a lot of knee bend/racquet drop/shoulder turn/back bend, which let him generate a lot of power without having to hit flat. Tactically copy the way he would rarely hit his biggest/flattest 145+mph first serve, instead favouring a more consistent 130mph spin serve. In other words: serve as big as you can without losing spin or consistency.
Isner/Karlovic/Janowicz: Technically -be tall-. Also, these players use a combination of heavy spin and high/forward contact points to be able to hit extreme angles, so hit the ball with spin at as high a point as you can consistently manage, while moving forward into the court to get open up angles. Tactically, lots of variation - sidespin, topspin, torpedo spin, placement, and power can all be changed - don't let them get comfortable with your serve.
Always hit big flat serves trying to be Roddick/Karlovic, as the big flat serve is inconsistent and easy for your opponent to predict.
Wimp out on serves, first serves should always be an attempt to get you in a favourable position, second serves must not be easy to attack - so really kick it!
Try to perfectly copy a pro technique, as you are different to them in terms of your body and training, instead you should take inspiration from what they do well and copy the general idea.
Wilson Juice 100s - Solinco Tour Bite 16L 50lbs - 4.5/5.0 topspin baseliner