Definitely my dad's mom, who lived just down the road from us. We visited her at least once a week, and she had a big impact on me until she passed away.
After that, my mom's mom, whom we visited in Japan for a summer when I was in elementary school and later when I studied abroad in college. A big part of why I studied Japanese was so that we could talk without a language barrier.
Which grandparent(s) were you closest to growing up?