Monk is probably the easiest, but they are all very similar in difficulty.
Monk gets hungry faster, but heals when he eats which cancels it out.
So effectively he just has 2 more health than the fighter who has no perks, from what I can see. And 4 more health than the wizard who can do a little more damage with summons, but summons are not great anyway.
Since Hunger and health are most important, Monk wins.