Pet Sematary (1989) on VHS at a sleepover at a friend's house. I was ~10, and it traumatized me. I re-watched it for the first time a few years ago, and I remembered every scene in the entire movie, in detail.
Do you remember what *your* first R-rated movie was? I finally asked my mom (I couldn't remember), and she told me it was The Lost Boys when I was roughly the same age as my daughter (about 9). The movie traumatized her more than it did me, apparently