Tough choice! My suggestions are:
For handgun: S&W Number 3, 2nd Russian Model (with the long trigger guard spur). They are a little strange to shoot, but they have the most beautiful lines!
Handgun Honorable Mention: 1908 Colt Pocket Hammerless
For single shot rifle: One of the Ruger Number 1 rifles with quarter rib sights. (they heavy barreled varmints are a little blocky!) But the 22" barreled version light weight barrel, No. 1 S or the 24" Tropical. (I sold my 375 H&H but still have my 7x57)
Rifle Honorable Mention: 1873 Officers model Trapdoor! The clunky actioned Infantry rifle is just ugly, but the Officers model was given very classic lines by the artisans at Springfield Arsenal.
Pump Shotgun: There is no such thing as a graceful pump shotgun! But some of the most memorable lines are on the Winchester 1897 with a 20" barrel.
Over/Under Shotgun: Browning Superpose Dania grade (20 gauge is best proportioned.) Being in the presence of one makes your heart race!
Best crew served machinegun: TIE between Browning water cooled 30 caliber, and the Russian version Water cooled Maxam Heavy 7.62 water cooled on wheels.
Best show stopping firearm: Gatling Gun, "Clip Fed" Brass frame, Police Model (on a pedestal)
Worst idea in a mass produced weapon: Nambu Anti-tank Rifle, strapped to the back of the assistant gunner!
Ivan