The problem with historicals is that they tend to be pretty specific in terms of who is fighting who. Of course, it's not an absolute limitation. At my store we recently ran the Romanian invasion of Finland and the Finish counter strike using Flames of War. I knew we shouldn't have taken that wrong turn at Abaqurerie.
Anyhow, Flames of War is the big name in WWII right now and pretty common place but I suspect Battlegroup from The Plastic Soldier Company is going to over take them due to better prices, better kits, and more detailed / realistic rules. The Fall of the Reich book is out this month and I think that will be the big turning point as it's a period more people play.
Warlord's Hail Ceaser, Pike and Shotte, Black Powder, and Bolt Action are all very accessible and well supported. They're all by Rick Priestly who designed Warhammer in the first place and he's a very competent designer.