In the last two days I've watched two movies, Resident Evil: Afterlife and Pandorum, that utilize amnesia to differing degrees of effectiveness as a plot device. This got me to thinking about how often this trope is used.
Amnesia can serve a few different functions. First of all it enables exposition quite well, as it explains why characters are asking so many questions and getting long winded, detailed answers. This can also render the character with amnesia as the Audience Identification Character, as we learn as they learn and therefore see through their eyes. At other times, however (such as RE: A) it is a cheap attempt at masking a shallow character with a provocative quirk.
With that in mind, what are the best usages of amnesia in movies? This of course will involve spoilers, so tread lightly.
One of the best? I'm reminded of the Tom Berenger starring Shattered (1991). In the movie Berenger wakes up in a hospital having been in a car accident. His face is horribly scarred, but after extensive surgery he's back to his old self. Slowly he discovers that his wife may have been having an affair, and that her and her lover may have been the ones that caused the car accident. At the end of the movie, however, (SPOILERS AHOY!) he discovers the dead body of none other than...Tom Berenger! Turns out he had been the wife's lover all along. Pretty awesome twist. (SPOILERS ENDO!)
Worst? Claire in RE: A, whose amnesia serves no purpose, never seems to clear up, and screams "Ali Larter wanted a bigger part in the movie!"