GMT games has put up a list of it's all-time 15 best selling games. It's pretty interesting!
One thing to notice is that 3 non-war games (Battle Line, Formula Motor Racing, and Ivanhoe) made the list (I've not tried any of them, unfortunately). Also, one expansion is on there: C&C Ancients Expansion #1 (also, never tried it).
Despite the fact I love wargames, I have played VERY few of these. Twilight Struggle I've played, and love. Combat Commander: Europe is something I've also played and enjoyed (some battle reports are posted on this blog, even). SPQR and Great Battles of Alexander are games I own, and would love to play, but haven't had the opportunity.
Sadly, that's all. Despite GMT's popularity, there are so many of their most popular games I've not even tried, nor own. It's a shame, really. Something to work on fixing over the upcoming years, no doubt.
There's also a list of their best sellers for the current year.