I just caught part of one of the newer Terminator movies as I was flicking through channels on TV, and I was only able to identify it because a scene with a visibly 60-plus Arnold Schwarzenegger made me realize he probably hasn't acted in a decade and a half otherwise. I've never seen any of the Terminator movies... a fact which never fails to elicit shocked responses in movie conversations. I've also never watched any of the Alien, Indiana Jones, or Harry Potter franchises, am yet to watch any of the Shreks (despite one of my exes quoting them all the time and telling me she basically learned most of her life lessons from them), nor any of the Toy Story movies. I think the only Pixar movie I've ever watched is Up. I do mix up Pixar and Dreamworks all the time, so that whole debate between the two goes over my head, but I know I've never found Nemo or seen Frozen, or practically any of the others.
Obviously this is most egregious for stuff that's been released during your lifetime, or maybe adulthood, but feel free to include whatever you've never been particularly convinced you've missed out on, from any era.