I'm suprised that Cesnna's ubiquitous 172 hasn't got a mention. Not the sexiest looking aeroplane out there, doesn't fly the nicest, nor is it the quickest, but for what it was originally designed to do it does it extraordinary well. Duanne Wallace got it pretty much correct right out of the box. Just like they did with the DC3 and the 747.
Those three aircraft would definitely make it onto the 'all time greatest' list.