A lot of those didn't seem so "unexpected."

The two that come to mind are PULP FICTION and BURN AFTER READING. (Both listed)

Both are similar. Unlike the majority of other "unexpected" deaths that occur in the last Act, or as a culmination that propels the last act, these two occur in the middle. They also seem out of the blue in terms of expectation.

In terms of story, they fit right in.

I'd say BURN AFTER READING hedges out PULP FICTION for unexpectedness, mainly because we, as an audience, have been trained to know that main characters, especially stars, will make it through the movie (or at least to the end when their death can be the most dramatic.

Which leads to their perfect number 1 choice -- PSYCHO.

PSYCHO is the forerunner for completely unexpected deaths. This is atypical from all other unexpected deaths, as it is the protagonist that dies half way through the film.