i tried all 3- H is the best, IMO, since the enemy doesnt bliz you as they do in VH, or try to at least, but they dont wait around and wait to be conquered like in M. dont even try E. its more like "sandbox" than anything else. my friend tried his first campain on E, but quit b/c it was too easy.
so my best bet, is if you want a challenge, do VH, but if you want to RP or are doing an AAR, do H.