Well, in the unobtainable class....Gloster Gladiator
In the nearly obtainable class....the Beaver. I've had two passenger trips in the Beaver (well 3 if I include a stopover at Saltspring Island en-route to Vancouver from Pat Bay). What an impressive lifter, even on floats. The sound and the feel of being airborne was visceral. I'd love to have a shot at flying one
As an aero engineer and a pilot, I have to say the Cessna 172 is a true design classic. Does exactly what it says on the tin. Had the opportunity of flying an amphibian version yesterday (off land and water) and it just solidified my impression of the design. A true classic
For the perverts amongst us, I think the Robinson R22 is a classic - well, a classic piece of design at least. Frank Robinson is an engineering genius
I haven't mentioned the Chipmunk - that's because I'm a part-owner of one. It's good; it's very good, even without inverted oil/fuel, vp prop and another 40 hp. But to my mind it's not a classic, just a very very nice little aeroplane that did its job extremely well