Well Ben it's a tricky question, opinions are like arseholes, everyone has got one, on the best club it all depends what you're looking for, for a long time I liked Samya, not a big club like the ones down in Hessen, but well run club, free beer, usually a good selection of girls.
I will still return there when in Cologne, but I also like Sharks, Oase, Palace if you dodge the sharks, and Artemis.
Highest number of girls: is probably Artemis or Palace, though Oase and Sharks also have many.
Atmosphere: has to be Samya, on a busy weekend night the lounge is a balkan disco.
Approach policy: traditionally the NRW club have a no approach, but it's not set in stone, Hessen and Artemis girls are proactive in approaching you.
Girl Quality: Difficult to judge, as everyone has slightly different taste, and these girls have a habit of moving around clubs, for model stunners I'd say Artemis, Palace, these two usually have the glam barbies with fake tits and tattoos, for more natural type Oase has more young Romanians.
Sharks: The award for the most sharky club in all of Germany goes to Palace, there is a hardcore of regular girls there, who have formed a hooker union, they think €50 isn't enough, and will give you a rushed 10 mins with brief oral and no kissing etc, they try to market their special service for €100 half hour, trouble is the club lets them do it, and there are so many clueless "I've never fucked a German hooker" type guys who visit the club that they will pay whatever they're told, but you can still get decent service.
Price: the price varies slightly, if you want cheapest bang for your buck, go to one of the cheap red carpet clubs in the Rhur, but quality of the girls might not be top draw, NRW has always been good for the punter, €50 first 30mins, then €25 for second 30mins, then €50 for third 30mins, so if you wanted 90mins with a girl it only cost €125 instead of €150 in Hessen.
Surroundings: Artemis has nice rooms, so does Oase and Oceans, in general Oase building and facilities is good, same as Sharks and Artemis, Planet Eden have great facilities with large garden, but it's a white elephant, hardly any visitors or girls.