Maybe a bit late but I've only just seen your post.
Best club is a difficult one as I don't know exactly what you are looking for, and one persons best club can be anothers hell hole.
Firstly there are no clubs at all in Dusseldorf city, to the best of my knowledge they are not allowed there.
In Dortmund the only swinger club is Schickeria which is just outside the city itself. It's a while since we were last there but we had some good nights.
If you are wanting couples only then Die Eule at Rattingen (close to Dusseldorf airport) is the only one that doesn't involve quite a drive. The only other one is Life Club but it is quite a distance away.
All the others are mixed (couples and singles)
For large European super club style then there is Caesars Palace at Sprockhovel. Very large on 3 floors with a superb outside area for the summer and probably the best buffet of all the clubs we have been to. On the downside it can be a bit more like an erotic disco than full on swinger club, but there is usually some fun to be had.
Another similar club is Steinenhaus (Stonehouse) at Hattingen. Again very large on 4 floors and can get 300 + people at the weekends. Great facilities, excellent buffet, and usually action packed.
There are many other clubs in the area that range from slightly smaller versions of the above (Faro 8 at Marl, Villa Palazzo at Dorsten for example) to pure no frills sex clubs (Blue Heaven at Gelsekirchen would be a good example) but they are not for the faint hearted.
Hope this helps.