I'd never thought about this before, so it kinda shocked me for a second. I think I know what I'd ask, though.
I'd want to know why He loves me. God, really, has no use for humans. He doesn't need us for anything. None of are perfect; none of us totally obey Him. (Well, of course, with the exception of Jesus, but Jesus is God in human form, so the question could be directed to Him to.) God could totally destroy everything - humans, earth, animals, ect. - and start completely over with a race that is totally devoted to Him. He doesn't choose to do that, though. He loves us, and what's more, He forgives us of our sinful ways. So, yeah, I'd ask Him, "Lord, why do You love me?"