You may remember the old This Week on the ATP Tour show on ESPN that was on Thursday? nights in the late 1990's......and had some British guy announcing it
I recall they once asked a number of top ten pros circa 1997 (Ivo, Rusedski, etc) who had the best second serve. I think the general consensus was Becker, Sampras and Stich not necessarily in that order. I remember the interviewed players mentioned that their serves were particularly difficult to read and/or had a lot of action (heavy spin).
I also recall that some magazine (Tennis Week?) published a study around 1998 that measured RPM's on second serves. Sampras had by far the highest RPM second serve. He had somewhere around 4000+ RPM while most other top pros were in the 3000 range.