Sampras has the complete serving package. Placement, speed, spin, consistency and disguise on both his first and second serves. His second serve really was a cut above the rest.
Federer is very close to this imo. Maybe better on the disguise though, although Sampras' serve looked cooler.
Interesting thing about racquet technology, wasn't there a test where they measured Mark Philippoussis' serve and then gave him an old wooden racquet and he served within 1-2 mph of the speed of the new ones?
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM