What is, in your opinion, the most obvious sequel hook (or hooks):
In my opinion is probably the Avengers movies.
I just watched Immortals last night on Netflix and the final 5 minutes are just one big preview for the next movie.
@VioletEyedDragon: I would HP and LOTR shouldn't count. Those sequels are teased because they are books, it isn't like you go into those films not knowing there will be a 2nd or 3rd.