Personally I believe Demo is the easiest of the three. Sure, Affliction and Destruction aren't exactly difficult to play in their own right but they have some mechanics that separate good warlocks from great ones, namely dot maintenance on multiple targets for Aff and fast-paced decision making with havoc for Destro. Both are pretty straightforward and intuitive but actually executing it well all of the time can be challenging. For demo, where is the challenge? Making sure you don't sit at the wrong angle on the bench and get a splinter?