Sampras mentioned that Stich was a player he feared. I don't have his book in front of me now but I got the impression he feared Stich more than anyone because he felt Stich had no weaknesses.
Overall Stich was so good in everything but as people have written, Goran had one of the greatest weapons in the history of tennis in his awesome serve so who is to say. Of the three I think it's a no brainer that Goran had the best serve.
Krajicek was also an excellent overall player and all of them had super serves.