Goran´s serve is the best effective one WHEN it works. As it is not very reliable, since he can get 20947183569235 aces in a row AS WELL AS that many double faults in a row, overall I cant say he has the best one.
Taylor Dent has LOADS of power, and is a POTENTIAL best server. But he is not consistent enough yet to make it a reliable weapon.
And I must agree with Lindsayfan and Daniel, Petes serve is great. Maybe you didnt see it when it was effective, but it was a major weapon. And a plus, it used to ALWAYS work.
Krajicek´s motion is great, and Rafter has the most beautiful serve.
Overall, I still get Sampras, since he is the one who had it working for longer, who won a lot of matches on that weapon, and had it consistently at his serve. Serving is not only blasting, it is mental preparation and a matter of picking the right choice. Pete did all that.