I am heading over to Frankfurt/Dusseldorf in order to sample the FKK experience for the first time in a couple weeks. I will be visiting both areas for 3-4 days, and focusing primarily on Oase, and Villa Vertigo, though I will probably visit one or two other well reviewed FKK's too.  
I am curious whether anyone knows of message boards, or forums, where I can get first hand accounts of girls who provide top notch service at Oase/ VV  vs those who look great but are not that concerned with service. I realize that the girls move around, however, I am trying to do some research and identify two or three "must see" type providers at both establishments. I have found a little bit of information on other sites, but think perhaps someone may know of a better way to do some research along this line before the trip. Any guidance would be appreciated.

If you can read German your best bet is one of the German discussion boards. Women change clubs, names
and your mileage may vary depending on chemistry. Years back I used to ask my guide for recommendations but after a while found that my gut feeling about a gal had more success. There are some things you can do when selecting the right gal. Watch her with other customers, note the aggressive ones tend to be not as good. Approach her, chat a bit and then tell her maybe later, her reaction will tell you volumes. Usually the diamonds in the ruff never approach and wait for you to come to them. Gals that are constantly going to the room are a good sign as well.

Recommendations will depend also on what you like ie Deep throat (deepest around) would be currently Viagra Brazilian gal very dark and enhanced look at Mainhattan club. Then how she looks may not appeal to you but her skill is second to none. While I've recommended women to many friends my chemistry with the gal can vary to theirs. Mara at Bernd's for example is well known for top service but she's not my cup of tea.
The problem with the discussion forum folk from the US is they find a star and tell the world, not too long after that she's changed. Then again this is a human business so anything can change and it does.

Some names that come to mind
Oase has it's long time super stars, Hana CZ blond, Linda GR Black, Judith HU GFE Brown, Gabriella VN dark, petite
World Goldmarie, Blond, larger chest well known
Bernd's Ana, Katja, Mara, Tanya, Natalie
Mainhattan with the exception of a few like Viagra many women from World