Historically I prefer the Northern front; Winter War, Continuation War, Norway, ect. However I find the winter scenes to be visually hard to play in SP, so I generally just muddle through them
In game, I'd have to say early Eastern front and USSR/Poland, nice scenery, good maps, and the German uber-toys are still rare, like they should be. But I must give honorable mention to the few Spanish Civil War scenarios out there, always a LOT of fun.
Matt