Probably KLM or Qantas - being the oldest airlines in the english and non english speaking world respecitvely, although QF has changed its logo format, it has consistently had a kangaroo included in it and I think that KLM has always had a crown as a part of the Royal Dutch Airlines livery!
Monarch's logo is really old, but the actual livery has changed a million times. American, yeah, they've had the same livery for quite a while!!! United have had the same logo for a long time, just different livery. Good luck!!