John Di Ponio
Pretty much depends what you want to have. They offer different challenges but all function basically alike.
If I remember correctly, Hasty Positions was the first, but it really doesn’t matter. Hasty Positiins does offer some Red Devils scenarios too. Not sure about the others off the top of my head.
Agreed with Michael. Purchase what interest you and then go from there. No sense overloading yourself. You can get use to how the X-Terrain works and then adding the other pieces becomes very easy. I was hesitant at first to use the X-Terrain but it adds so much to the game. I am in the process of purchasing seconds of some to make bigger games with more terrain.