A gander on the list of most [produced aircraft on wikipedia][1] suggests the [cessna 172][2] which started production in 1956 and is still produced today.

However the [Aeronca Model 7 Champion][3] has returned to production in 2007 after a short production run from 1945-1950.


  [1]: http://en.wikipedia.org/wiki/List_of_most-produced_aircraft
  [2]: http://en.wikipedia.org/wiki/Cessna_172
  [3]: http://en.wikipedia.org/wiki/Aeronca_Champion