If raw sales mean anything, the most popular army are Imperial Space Marines, followed rather closely by Tau and Chaos Space Marines. The others have surprisingly even sales, with daemonhunters, witchhunters, and Dark eldar bottoming out the list (in no particular order) due to either mostly metal models or difficulty to get the models, as is the case with Dark Eldar. That being said, Witchhunters and Dark Eldar are both extremely potent in the game, so it's really more of an issue of the models than it is lack of interest, I think.
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