How do you want to rate popularity? By sheer number of posts, and considering only Saladin and later, Sally was most popular at 174 posts and Washington second at 171. Willem is only up to 127. Some of Frederick's were about the problems with the postings, though, and those sorts of things ought not to count (but are too much work to keep track of).
The most messages of all time was for Game IV (Frederick) at 239.
But that's only a crude estimator of popularity; it should really depend on what people are talking about and how enthusiastic they are about the game. Possibly also the number of people who play -- which I meant to keep track of in all those "reserved" 2nd messages, but have never really had the energy for.