Chipmunks
For me it was one particular flight in a Chipmunk whilst an Air Cadet. It was the last flight of the day, I'd had a bit of a pole around and we'd just turned round to return to the airfield when the pilot said "Want to do some aeros?" I said "Yes, please" so we did barrel rolls all the way home using the setting sun as a reference point! Been hooked ever since...