Note: This list is completely subjective, and is based on how I think players will be remembered.
Please Read Criteria:
Number of Slams first.*
Slam finals second.
Ranking is third.
Non-slam career titles fourth.
Doubles titles serve as a tiebreak, when all else is even.
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Updated 1/28/13.
Top 20 Active Players
And on a DIFFERENT PLANE OF EXISTENCE...
1.
Serena.
31 USA

1 Career Golden Slam, 11 other Slams, 4 other Slam finals, 13 GS Doubles, 2 GS Mixed, 3 Olympic Doubles Gold Medals, 33 other singles, 4 other doubles, 123 weeks at No. 1. YE No. 1 player: 2009, 2002.
The Mere Earthlings...
2. Venus Williams 32 USA

7 Slams, 1 Olympic Gold, 7 other Slam finals, 13 GS Doubles, 2 GS Mixed, 3 Olympic Doubles Gold, 36 other singles, 5 other doubles, 11 weeks at No. 1.
3. Maria Sharapova, 25 RUS

1 Career Slam, 1 Olympic Silver, 3 other Slam finals, 23 other singles, 3 doubles, HR No. 1, 21 weeks.
4. Victoria Azarenka, 23 BLR
2 Slams, 1 other Slam final, 1 Olympic Bronze, 1 Olympic Mixed Gold, 2 GS Mixed, 13 singles, 5 doubles, hr. #1 for 49 weeks. Year ending #1 player, 2012.
5. Svetlana Kuznetsova, 27 RUS

2 Slams, 2 other Slam Finals, 2 Slam Doubles, 11 other singles, 12 other doubles, hr. # 2
6. Ana Ivanovic, 24 SRB

1 Slam, 2 other GS finals, 10 other singles, hr. #1, 12 weeks.
7. Li Na, 30, CHN
1 Slam, 2 other Slam final, 6 other singles, 2 doubles, hr. #4.
8. Sam Stosur, 28 AUS

1 Slam, 1 Slam final, 2 Slam Doubles, 2 Slam Mixed Doubles, 2 singles, 20 other doubles, hr #4.
9. Francesca Schiavone, 31 ITA

1 Slam, 1 Slam final, 4 other singles, 7 doubles, hr. #4.
10. Petra Kvitova, 22 CZE
1 Slam, 8 singles, hr. #2
11. Caroline Wozniacki, 22 DEN

1 Slam final, 20 singles, 2 doubles, hr. #1 for 67 weeks. Year ending #1 player, 2010, 2011.
12. Jelena Jankovic, 27 SRB

1 Slam final, 1 GS Mixed, 11 singles, 1 doubles, hr. #1 for 19 weeks, year-ending #1 player: 2008.
13. Vera Zvonareva, 28 RUS

2 Slam finals, 1 Olympic bronze, 2 Slam doubles, 2 Slam mixed, 12 singles, 13 other doubles, hr. #2.
14. Agnieszka Radwanska, 23 POL
1 GS Final, 12 singles, 2 doubles, hr. #2
15. Sara Errani, 25 ITA
1 Slam final, 2 Slam Doubles, 7 singles, 13 other doubles, hr. #6
16. Marion Bartoli, 28 FRA

1 GS Final, 7 singles, 3 doubles, hr. #7
17. Kimiko Date Krumm, 43 JPN

3 GS semis, 8 singles, 3 doubles, hr. #4
18. Nadia Petrova, 30 RUS

2 GS Semis, 12 singles, 1 Olympic doubles bronze, 15 doubles, hr. #3
19. Angelique Kerber, 24 GER

2 GS Semis, 2 singles, high rank #5.
20. Jelena Dokic, 29 AUS
1 Slam semi, 6 singles, 4 doubles, hr. #4.