I'd say Crucio, to weaken whoever I'm using it against, because Imperio can be fought with determination. And if I kill my... uh... victim, I can't get any information out of them. So Crucio is perfect.
Well, since my پسندیدہ character (Snape) used it on Dumbledore, not that I hate him, I am going to have to go with Avada Kadavra. What I don't get is why didn't Harry kill Voldemort when he first heard of the spell.
Imperio. I don't see what's not attractive about getting someone to do exactly what آپ want. =) Then Avada Kedavra- quick kill. I'm a faint-hearted person. I don't find the idea of torturing someone very appealing.