The Imperius Curse is the strongest Unforgivable Curse and all the other ones are just versions thereof. Just think about it - not being in control of yourself is torture as it is something you've been able to do your whole life. People freak out when they're not in control of themselves or a situation they may face. And the killing curse? That's also just another version of the Imperius Curse because when you have total control over someone else and their actions, you can make them kill themselves just as easily and just as quickly.

Life in Azkaban sounds like a pretty fair sentence as far as punishment goes for any of the curses and there's no need for a dementor's kiss. After a while, they'll probably be begging for one as they'll be going insane in Azkaban either way.