I have for a long time thought that whoever abducted Madeleine, they most likely acted alone.
For all the convoluted theories about the parents being the culprits, a VIP paedo ring being involved, the numerous sightings of MM around the globe etc etc etc, I can't help feeling the answer is going to be something rather simple.
I know it's probably confirmation bias on my part, but Christian B absolutely fits the profile. Yeah, he had criminal associates, but that seems to be related to lower level drug dealing and burglaries. As far as I know, the sexual crimes he committed were all done on his own.