Interesting topic :D
To me, the Cruciatus Curse has always been worst.The Imperius Curse can be forgiven if it is used to keep someone from doing something horrible or to aid someone in doing something good. Doing that would not have ill effects on the cursed person. Though, if it is used for evil purposes, it is an invasion of the human mind, which can sometimes be necessary.
IMO, Avada Kedavra is the least worst of them -- since death is not the worst thing that can happen; a moral standpoint which was reiterated by DD for the majority of the books. While I agree with DD's non-lethal curse method, I can understand the use in dire situations (I, for one, wouldn't hesitate in using the AK curse against LV).
However, the Cruciatus Curse is not acceptable under any circumstances. I don't give two hoots for why anyone uses the cruciatus curse: it is unacceptable in every which circumstance as it is inflicting pain for pain's sake. The act itself is savage and barbaric and there's no excuse for the one who fired the spell. This is precisely why I don't approve of HP's usage of that spell on Carrow. In that instance, I really hated him as I don't think anyone should derive pleasure from torturing someone else (no matter how awful the victim is).
The consequences of the Imperious curse can be rationalized and intellectualized to a certain degree or fully, depending on the individual, since the individual itself wasn't consciously involved in the act (supposing A imperio's B to commit murder). However, being crucio'd leaves deep scars - both psychological and physical - and I think the pain is far more difficult to deal with than battling with your morals and conscience since pain can manifest itself into dreams and so on.