I have a particular interest in railway guns. But I will assume that the OP meant rifles and other small firearms
No4Mk1(T), I have to agree with a few others.
If it wasn't for its forward-bent bolt handle, the French MAS would also get my vote; not sure why they are not appreciated more.
Light machine gun? I like the look of the Lewis. The magazine kind looks like a hat; stylish, classy accoutrement. "What, we're going into combat? Wait, I can't show up without something on my head, that would be a fashion faux pas".
The BAR also has some style, simple and clean, purpose-looking. Oh and the MG42, something about that one... ok sorry I'm steering off-course.
SMG? Easy:
http://2.bp.########.com/-pYlJjC7S4Zc/T-Y9T2tmJcI/AAAAAAAAAk4/JeZ9wFqf7OA/s1600/SoldierwTommy.jpg
Pistol? Maybe the Browning HP, the Colt 1911... No, the Luger I think. It'd be easier to choose the ugliest pistol, now that there are plenty of!