Of the current crop of women Serena is the clear standout of the crowd. Her serve is her main weapon, her signature. Her ability to hit 2nd serve aces and just flat out be aggressive with it is amazing.
Of past players. Nav had a solid 2nd serve, problem was she often got to defensive after it and it was ruined. He could place it well though.
Davenports 2nd serves were OK. She wasn't afraid to hit them....but against other power hitters she tended to go flukey on 2nd serve points.