jul 22, 2011 - Captain America: The First Avenger (July 22, 2011)
The first Captain America release
serves as one big flashback scene,
so arguably it could be first, or it
could slide in before The Avengers.
This is all a matter of preference
since the final scene shows Steve
Rogers (Chris Evans) awakening
from his 70-year slumber.
Added to timeline:
S.S Avengers Timeline