i'm going to narrow it to first serves, that i got to see on a consisent basis:
1. goran, no question. he got the ball so far from guys, he may as well have been serving to the opposite service box.
2. three way tie! stich, forget, krajicek.
plenty of other great guys, but these are the ones that stand out in my mind.
edit: just for the record, overall serve goes to pete. no big surprise, but when you factor in the 2nd, along with repeatability over time, and clutch moments, i don't know how i could pick anybody else.