Hmmm...interesting question! It is funny you mentioned CoA...in my experience, they are actually one of the most popular factions. When they first hit the shelves, they had some really good things going for them; cool models, unique weapons and abilities, and the fact that they were something "different" (ie, not in the core rulebook). While their popularity has probably died down somewhat, I would still put them towards the top. As for the least popular, I'm going to go with probably the French and/or the Russians. I think this is partly due to them having been released later, but also because (in America, at least) there tends to be less interest in playing as those nations.
