It's funny to differently people understand the question for the "easiest" class. Many seem to understand that as the "strongest" class. But in pure difficulty...:
Demo -> can be played at about 80% efficiancy with two buttons (tab, Shockstrike)
Necro -> may be more versatile, but requires much, much more buttons to reach a similar level of efficiancy
Combo classes -> needs Combos, i. e. 5-7 keys (tab, combo, 1 to 5 directional attacks) at least, and will be extremely poor if played that way