I know this is the military forum but - forgive me, I'm a civilian - what for?
For sheer elegance:
Concorde
Vulcan
Mosquito (thanks, Captain)
As a passenger:
A380 (though as derjodel says, it's still in service)
747-400 upper deck business class
Short Empire (even I am not old enough to have flown in one, but the idea of having time to take ten leisurely days to fly to Australia, stopping overnight in exotic places, is surely attractive).
As a pilot:
I was only a humble PPL, but for sheer fun, the best flying I had was in STOL aircraft like the Helio Courier, in and out of back-country airstrips around the world, often excitingly short and in really interesting places.