As much as I hate to pass up the opportunity to meet some people I'd love to meet and the chance to see my maternal grandma again... I'd say they would be my paternal grandparents and my maternal grandfather. I lost all 3 of them in less than a year when I was ~5 yrs old... So, despite remembering what they look like (mostly thru pics probably) and some very faint memories, I never got even a little chance to know them. I would like that chance.
If you could only ask one question, but would get a definitive answer... What would you ask?