I think weapons look the best when they are unmodified. I don’t know why, but I like the rather simplistic / sleek look.
That means preferably iron sights, no drum or extended mags (at a certain size they look silly and with pistols they ruin the sleek form by sticking out), not 100 rails with different things like flash lights and laser pointers added etc.
Some weapons look good with scopes, like a good old Remington 700. But a revolver with a big ass sniper scope attached just ruins the whole beautiful form. Looks a bit like a kid with a oversized hard hat.
In videogames I always rather pick the ‘worse’ versions of a weapon in favor of its looks. I think a 1911 with a extended mag sticking out and a sniper scope attached just looks horrible.
One notable example is the AK47 – its vanilla form is just a beautiful simplistic combination of wood and steel. Effective and reliable.