Although I agree that all the preceding aircraft are very fine in their own way, it has to be the 747 for me:
- The risk taken at the time; the next biggest aircraft was about 200-250 seats (DC8 Stretch) and here was a new aircraft that was nearly twice the capacity; Boeing bet the company and fought through so many odds to get it flying - from bad weather to early reliability problems with engines.
- The first widebody jet, first with "big fan"/high bypass ratio engines.
- Captured the imagination like no other aircraft (except perhaps Concorde); most people can't tell a DC10 from an L1011 or a 737 from an A320, but they'll recognise the 747.
- Still, after 30+ years, the biggest selling widebody of them all - c.1500 sold and still selling (expect Emirates 747-8I order in Dubai!)
- For much of its career, in various models, particularly the SP and 744, THE long haul leader; how many routes did it pioneer - nonstops across the Pacific, trans-siberian, europe to Asia.
On a personal level, I never tire of seeing a 747 land/take off. Long may it fly.