Necromancer. Necromancer's are as essential to anything as monks, except the only thing is that often times, bad necros = bad reputation = necros get thought of last. But any quest or mission can be made easy with a good necro, or even a half decent one.
Technically, I consider monks/necros tied for first, so second best player would be a really solid dervish. Their high damage aoe scythe and tanking/self healing abilities make them far better picks than warriors when it comes to tougher quests/missions.
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