my list isn't for the best player of the category, but for the players who use said shot as a major weapon:
Serve (first and second combined): Ivo Karlovic
, his first serve is too good that it compensates for his second serve liability, I wanted to use Sampras here but I had to use him for running forehand.
Return of Serve: Andre Agassi
(also James Blake).
Ball Control (changing directions of the ball): David Nalbandian
. (also Andy Murray)
Inside-Out forehand: David Ferrer
, pretty much his only weapon.
Down the line forehand: Igor Andreev
, scary forehand.
Cross-court Forehand: Roger Federer
, beyond mastery.
Running Forehand: Pete Sampras
, no competition.
Forehand Slice: Fabrice Santoro
(you should have added this one
Backhand down the line: Novak Djokovic
(also Nalbandian and Safin).
Backhand Cross-court: Andy Murray
Backhand Slice: Roger Federer
(simply you can not put anyone else here).
Lob: Lleyton Hewitt
Drop Shot: Thomaz Bellucci
is using it pretty well lately
Passing Shot: Rafael Nadal
Approach Shot: Andy Roddick
I don't know who has it as a major weapon
I can't believe you haven't put a category for volleys, oh well no one use them as a major weapon recently anyway (except Steps maybe).