when she is playing well, Serena, no doubt, perfect motion, good disguise, power, placement, and she can be pretty consistent, in a match against Hingis she did not miss one serve in an entire set
, also her second serve is pretty good.
Maria has an excellent serve, a very smooth motion, very technical, she uses her height and her legs to give it placement, and she serves to the four corners.
Davenport's is very similar (in fact, Maria's serve was modelled after Davenport's), so similar comments for hers.
Ivanovic's serve is pretty good, good placement and disguise and good power (but not massive), she needs a little more variety.
Justine's can be a real weapon, very powerful and goes for the line, but it's sort of predictable and way too inconsistent.
Venus's is powerful, but can be very inconsistent and her second serve is a liability.
Molik's is pretty good, especially her ability to hit a kick serve, a typical Australian serve, but it's NOT the best in women's tennis.