On the premise that 20 million members should mean that each card has ratings from a very large number of members (10,000 at a guess), here are the top 10 girls on median ratings:
As of the last week in June 2014. KEY: Name, 1st card in set/sets, median score, top score.

1. Little Caprice, 1312, 9.61, 9.65 *
2. Melisa, 581 689, 9.58, 9.68
3. Viola, 1517 1561, 9.58, 9.65
4. Anna Tatu, 1032 1549, 9.58, 9.65
These top 4 girls are the only ones with cards rated 9.6 or above, and each of them has at least 3 such cards. In view of this, I personally consider the 4 girls above to be equal top ranked.
5. Eve Angel, 1038 1068, 9.54, 9.56
6. Jana Cova, 751, 9.54, 9.56 (only 3 cards) **
7. Zafira (post boob job), 776, 9.54, 9.56 (only 2 cards) ***
8. Mandy Dee, 827, 9.53, 9.56
9. Domenika C, 1336, 9.53, 9.56
10. Antonia, 966, 9.53, 9.54

It is my feeling that many of the earliest VGHD cards are under-rated. Possibly due to the earlier cards having a higher percentage of clips showing no nudity.
In fact, of the top 25 median scores, only 3 girls (Melisa, Jana Cova & Morgane) have card numbers below 600.
At the other end, only 4 of the lowest 25 median scores have card numbers above 600.
My figures do not include cards after no.1580 as I don't have complete sets for the later releases.

• Little Caprice has the fewest published cards of the top 4 girls.

** Jana Cova's earlier set (560) reduces her median score to 9.44 and approx. 20th place.
*** Zafira's earlier set (294) was from before her boob job, and not included.

All cards' ratings are liable to change as new members post ratings. The later releases can change dramatically quite quickly due to lower numbers of posted ratings.

For those interested, the next 15 girls ranked by median scores are:

11. Cindy Dollar, 893, 9.51, 9.57
12. Marica Hase, 1500, 9.51, 9.55
13. Playful *****, 759, 9.50, 9.57
14. Aleska Diamond, 914, 9.50, 9.53
15. Angel Kiss, 666, 9.47, 9.54
16. Abigail Johnson, 722, 9.47, 9.51
17. Kami, 704 960, 9.46, 9.53
18. Nici, 1511, 9.46, 9.53
19. Robin, 1259, 9.46, 9.50
20. Gracy-*****, 1104, 9.46, 9.49
21. Tess Lyndon, 1052, 9.45, 9.54
22. Jana Cova, 560 751, 9.44, 9.56 If we disqualify her from top 10
23. Vanda Lust, 1449, 9.44, 9.53
24. Morgane**, 566, 9.43, 9.43 (**Median drops to 9.32 if set 456- is included)
25. Silvie Deluxe , 754, 9.43, 9.47 ( only 2 cards)

Also with median scores of 9.40 and above, in alphabetical order, are:
Angelica Kitten, Gina Devine, Isizzu, Kathy, Linet A, Lorena, Mia Sollis, Miela, Monika,
Paula Shy, Sophie Lynx, Valentina, Zara

I want to give honorable mention to the following girls, all of whom, IMHO, are under-rated:
Ashley Bulgari, Eufrat, Ariel, Jenni Gregg, Kitty Jane, Jewel, Nikita, Lola, Viki, Jasmine Andreas, Carmen Gemini, Samantha Lee, Zuzana, Sandra H, Gwen, Vicky S, Danielle Maye, Monica Scott, Chikita, Jana H, Chems, Marie, Nikita Valentin, Suzie Q & Shabba (for superior skills on the pole), Karina (best dancer), Zoe (could be Eufrat's twin), and the most seriously under-valued - Dorothy Bank. Several in this last list would be in my personal top 20.

Great research "ozbomber"! Thank you

In the 3 months since my research was posted there has appeared another contender. As of September 28th, Naomi Nevena has at least 4 cards rated over 9.6. My collection is not complete, but her 5 cards I DO have produce a median score of 9.62 and a top score of 9.65. If these scores are maintained over time, Naomi would edge out Little Caprice, Viola, Melisa and Anna Tatu from top ranking. But the 5 of them are so close as to make it pointless to differentiate.
As far as I can tell, these five are the only girls to have cards rated above 9.6. and all five have at least 3 such cards.