Well trying to list only one plane is very very hard to do. How about listing one for each type of category? Maybe by size and type?
For small 1/2a planes it is very hard to beat an Ace Mach None. Simple and a great flying plane.
In the 20 size I would say it was an Airtronics Jetfire 20. Similar to a Ugly stick but a little more refined. Fantastic flying plane.
In the 40 sized I would say the Goldberg Jr Tiger has to be the all time best plane. Well for me any way.
In the 60 sized I would say the Goldberg Super Chipmunk. Also the Tiger 60. There are so many great flying planes in this size. Would be very hard to limit to just one.
I have 2 gliders that I would have to list together. The Marks Models/Dynaflite Wanderer and the Dynaflite Talon. Both were great flying planes.
Buzz.