This random question has been floating around in my mind for the last fifteen minutes, so I thought I'd put it to a poll:
Poll Aunt Petunia's house Ravenclaw.
Why?
Well, from what we see, she openly dislikes the magical world - whether because of jealousy, fear, or whatever - but she can't seem to help finding out more about it.
She spied on Lily and Sev and listened to their conversation and remembered the details twenty years later. That's pretty impressive.
She's always described as nosey and interested in other people's business, which might indicate a natural inquisitiveness in her character.
I see her as needing to understand things around her, which points to a Ravenclaw frame of mind to me.
What do you think? Like I said, I only started thinking about this tonight, so I haven't a very fixed idea in my mind. Feel free to tell me if you think I'm wrong about 'Tuney. You could still convince me she belongs elsewhere! :)
(Though I'm fairly sure she wouldn't be a Hufflepuff.)