The visualization below shows how often the original Avengers have appeared in MCU movies together since Iron Man in 2008 and counting up to Thor: Ragnarok scheduled in 2017. The thicker a line, the more movies in which both characters appeared.

Because all six have appeared in both Avengers movies, the minimum number of co-appearances is 2. Iron Man and Black Widow are the Avengers who have appeared most often together, with 5 movies.