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A suggested order for conditions, from least to most serious:

Marked (the condition alone - it becomes more problematic when the attacker has related abilities. But you could say that about most of these)
Deafened (the condition alone - but often has a long duration)
Prone
Slowed
Grants combat advantage
Dazed
Weakened
Immobilized
Restrained
Blinded (the condition alone - but often has a long duration)
Surprised
Stunned
Dominated
Petrified (depends on what level your companions are)
Unconscious
Dying

It's not really possible to rank Helpless because you're hardly ever Helpless alone, but only as a result of being Unconscious.