Mmmmmm, that's one of my faves too. I suppose the Airsporter Mk 1 must be right up there for classic lines (and like the Cadets...it looks "right", and this initial feeling is confirmed by the actual balance and point-ability of these guns as you shoulder them)
From a "nuts and bolts" perspective...ie purposeful / engineering look .....Westley Richards "highest possible" with the heart cut out in pistol, Webley Service in rifle are high on my list, although I prefer BSA's as shooting tools.......The original Lincoln and pre 1914 underlevers are also very clean looking and uncluttered, although not to everyone's taste.
The Original/Diana mod 50 must be right up there as well, and the Webley Mk 111...