E.T.
As for Citizen Kane, the most impressive thing about it is that it is totally the work of an auteur, and a young one at that. It also brought to mainstream Hollywood a number of film making techniques that hadn't really been mainstreamed. And there's the whole controversy about getting it made and released at all. For the most part, I think it holds up well, too. Perhaps not as well as Casablanca, but then Casablanca benefits from being a timeless bittersweet romance set within the context of World War 2, so the whole thing just feels, well, timeless.