In deciding where to cast my vote, I had to think hard on what "greatest civil aircraft" actually means.
And quite early on I decided that, nice though it is, the Dakota can't qualify, because a lot of it's reputation are from military variants (the C-47 Skytrain and C-53 Skytrooper, the R4Ds, the AC-47, EC-47, VC-47, the C-48, C-49, C-50, C-52, C-68, C-117 etc, and of course the Dakota). The DC-3 was the civil type, and while it was important, it didn't achieve greatness, at least not without it's military and ex-military siblings...
So that leaves, for me, the Boeings 707 and 747, and the Concorde. The 707 fails, I think, on the basis that it didn't do anything new, rather it did it (perhaps) better and certainly in greater numbers than the Comets and Tu-104s and their successors.
In a different vein, the 747 is ubiquitous, but is it really great? Looking at cars, I'd agree that the Mini and the VW Beetle and a few others are great designs, and they were great commercial successes, but to call them a great car seems to miss something. The 747 (and, indeed, the 707 before it) did wonderful things for air travel, but to reduce everything to the level of "the cheapest is best", which requires the soul of an accountant to truly believe.
So that leaves us, more or less by default, with the Concorde. The design is as iconic as that of the 747; the engineering is (I feel) more impressive than that of the Boeings, and certainly that of the DC-3. The impact on people's imagination was pervasive.
Plus you could take members of the Flat Earth Society up in the thing and say "Hey, look, the bugger's round after all!".
It's good to see G-BOAG sitting next to the first 747...
Malc.