I guess a simple answer is, that if I shut my eyes and think of a typewriter, I see an Underwood. 

To put more flesh on that, you can see all the elements of a typewriter laid out in an Underwood: the spools stand proud, the typebars are exposed, and the black/red ribbon runs across it like a vein. The neat, compact design is important too; although desktop models interest me, they have a plodding, primitive, dinosaur feel about them - the Underwoods look whole; as if they've 'arrived'. The colour is also a factor, the glossy black contrasting with silvered chrome and white keys. Although I love coloured typewriters, the classic colour scheme, for me, has to be black and chrome.

Other makes and models might have the same basic elements, but none of them seem to fit together in quite the same way.