Panzer Corps. Easy to learn, and easy to pickup when you haven't played for awhile. It has a great system for choosing difficulty levels and good tutorial. There is going to be lots of DLC that will help defray the cost of keeping the game up to date and fresh. I'm just a big kid, and like pushing around toy soldiers in the sandbox.
ATG, is a good game with TONS of scenarios. My only major complaints are the tutorial is lousy, and there is way to much micro management, and some issues with oil, but that is why we have choices in games.
You might look at Warsaw to Paris. It has a newer UI, that might be easier to deal with, but I don't own that game.
I have ATG, and I don't find myself playing it much, but I'm sure LarryP will completely disagree with me.