sci fi and fantasy are likely the most popular. just look at what you actually find in stores. if you go to any FLGS in the US or UK you are going to find a lot of gw stuff, reaper stuff, star wars games etc.. you rarely see historicals,
if you wanted to know what game is most popular i have no idea. but i have a good idea as to the genres
like isaid, first its probably sci fi, then fantasy-but in the 80s and 90s i would have said it was the other way round, with fantasy being the bigger. then its the historicals. I would say in that over all genre, napoleonics and ww2 are likely the most popular. WW2 maybe being bigger with games like bolt action being pretty big, but im not 100% those are two genres i have only passing interest in.
then i would say ancients, those are pretty damn popular with a lot of games coming out. Thats another way to tell, you have a lot of new games for those three genres.
then i would have to say stuff like 7tv and where heros dare and pulp alley.. that sort of o movie based stuff that really only gets a lot of play with historical players(or so it seems, ive never seen a pulp game at a regular gaming con that i didnt run myself, but i see them all the time at historical cons.. i think its because stuffy historcal wargamers can convince themseles its not sci fi/fantasy becasue its loosly based on history)