My grandparents. I only ever knew my grandmother on my dad's side; my grandfather on my dad's side died when I was only a couple months old, and both grandparents on my mom's side died before I was born. I'd just want to get to know them.
Other than them, I'd say maybe God, if there is one. Just to see if there's anything I need to make up for that I may not be aware of.
I want to talk to my grandfather (my mothers father). I never met him, and I look up to him constantly. He seems like such a great man and it makes me sad that I never could of been with him, even for a short time.