Avatar: Exteremly simple. The popularity of the movie is tied with its biggest criticism--, that it's a very simply, tried and true formula, seen in everything from Dances With Wolves to Fern Gully. Translating it from other space to some kind of fairy land would be no trouble at all. A bunch of sparkly, nature-loving people riding dragons and defending themselves from heavily armed oppressors would certainly translate. The question of course, is how the hero would find himself in a navi body, but stories like this usually don't sweat those details.
Titanic: Obviously, this story would be told amongst generations as something akin to the Atlantis myth. The Titanic would be talked about even without the influence of the movie, but the star-crossed love story would also be very enduring. It might be interesting to see plays staged with "I'm king of the world" passed on with the same familiarity as "y tu, Brutus".
Avengers: Very simple--it's a story of the world's greatest heroes teaming up against does which are not dissimilar to ogres and dragons. Captain America, Iron Man and Hawkeye have very translatable medeival counterparts, and I've talked before about how the Hulk could be worked in. I don't know if Thor would be used as a character, due to his pagan nature, but some fine-tuning/replacing him with another hero would be simple. I can actually see this being a case of more and more heroes being added to the narrative as centuries go on. In times like this, character development and plot schematics are less important than it just being exciting.
The Dark Knight: More well-plotted than the Avengers, perhaps too much so, I can't see the events of this movie being staged at a play or told around campfires. The premise of Baman, and that he would battle someone called "The Joker", that could easily happen. The Batman myths in general could be spun into something new, but the particular plot of this, less likely.
Star Wars: Star Wars draws on so many fairy tale conceits, that the only major difference is, that it takes place IN SPACE. Therefore, one is left to wonder how much would be intact, and how much would be scavenged. Yoda, Darth Vader, they're all large parts of the cultural fabric. The battle between Luke Skywalker and Darth Vader culminating in the revelation as his father is definitely remembered (it should be mentioned that scene is in neither of the Star Wars movies that are on the top ten list. perhaps I should talk about franchises?)
ET: interestingly, ET has kind of disappeared from the pular consciousness recently, it almost invokes the 80's as a decade. Likewise, I think a part of it has kind of slipped through the cracks,man's it's not bombastic enough to be one of the quintessential "b-movies", but it's not a straight drama either, so it's probably rather orphaned, fanbase-wise. Also, like with Star Wars, there's the "alien" factor,