My vote would be for the Ford 9N with the runner-up being the Caterpillar 60 bulldozer. I say this more in terms of being tractors that are highly recognizable, and photographed.
I really think the Ford 9N gets the prize though, only because it was so popular in the 1950's, that they made a second Boomer Version of it 50 years later. While it is not my intent in this post to argue for or against the merits of the latter tractor (it was a different era of tractor making), how many tractor models do you know of, that have been resurrected after 50 years? It does say a lot about nostalgia...
Now do not get me wrong, because I love my Kubota and its capabilities, but in terms of sex appeal, personally...and this is just my opinion...I would rather see a pretty woman, in an abbreviated dress beside the old Ford 9N then I would my Kubota or other modern tractor. And I think the fact that I would love to see that combination, and have seen a few women who did model beside an old 9N, and personally feel the photos looked nice; that I am able to make a strong point regarding the Ford 9N.