I always loved the Needler from the Halo series. It was a really unique design at the time and probably the most fun I've had with any weapon in a first person shooter. Also, the explosions and the ching it does when reloading are just perfect.
Deckard’s gun in Blade Runner was always my favorite. Somewhat uncanny because it was clearly sci-fi, but retained that hard boiled detective aesthetic, like you misremembered a noir shootout.
I have way too little knowledge about the genre in general but there was something oddly satisfying about the Service Weapona.k.a.Director’s Gun from the Federal Bureau od Control..
But I am guessing the part of using it as a game mechanic makes a big difference, there were plenty cool looking things in movies or series — but this one just jumped first to my mind.
Halo's OG AR, the MA5B has that utilitarian look to it, like the rest of that universe. I like that because it feels realistic in a way (think of the utilitarian designs to the armor and vehicles in Aliens, for example.)
The railguns in Eraser, with their oversized x-ray scopes. Nothing special in retrospect, but I thought they were awesome when I saw the movie in theaters.
Weta Workshop has put out some cool-looking stuff. The Chemrail in Elysium for instance. Although, I think I'm more enamored with the in-universe lore of it. From the fandom page:
It is a dual-stage weapon, use chemical propellant first and then electromagnetic rails as the second stage.
Pretty cool actuation! That and the Lightning Gun from Unreal Tournament 2003/2004 are the only two with memorable (lore-wise) firing mechanisms for me. Here's the Lightning Gun's:
Once the target has been acquired, the operator depresses the trigger, painting a proton "patch" on the target. Milliseconds later the rifle emits a high voltage arc of electricity, which seeks out the charge differential and annihilates the target.
Design-wise though, I think Weta's weapons in District 9 were more appealing—more exotic, u noe? E.g., the ARC gun, and the Repeater.
I think the District 9 designs must have influenced the weapon design in the Titanfall franchise too, and I like the weapon designs there as well. The Flatline is maybe my favorite pilot weapon design (not to use though). Charge Rifle is pretty cool too though, with the 'breathing' capacitor banks on the sides. Titan-wise, gotta be Tone's cannon, hands down.
I'm rather fond of Murphy's burst-fire pistol and hide-away leg holster in Robocop. If we are talking the coolest gun, it's got to be the Zorg ZF-1 from the Fifth Element. https://grabcad.com/library/zorg-zf-1-with-shutters-1
I want to toss the Type-2 phaser from Star Trek (circa 24th century) as my personal favorite. It has this design which reads as a tool more than a weapon, which I love.
I guarantee you've never heard of it because it only appears in a series of novels by Steve Perry, the Matador books.
The gag is, it glues to the back of your hand. The barrel runs along your index finger, and you fire it by pointing your finger and touching the barrel tip with your fingernail.
It fires microdarts which can have a variety of effects, the #1 use being a drug called "Spasm" which makes every muscle in the body contract involuntarily, leaving the victim tied in a knot for 6 months.
The original trilogy of books:
The Man Who Never Missed
Matadora
The Machiavelli Interface
From a novel so you don’t get to actually see it, but the clockwork diamond pistol from Chasm City always caught my imagination. Mostly transparent and needs to be key-wound.
I was an eldar player back when I played WH40k, but the tau railguns are such a great bit of visual design. Perhaps amplified by how terrifying they are to be on the receiving end of.
I'll take the boring route. I respect the gun design of the N7 weapons. They look like futuristic versions of our current weapons but the paint design is on point.
Two of my favorites are from books and don't have pictures: the nanotech weapon given to grunts in "Old Man's War" and the Soft Weapon from Niven's short story titled, appropriately enough, "The Soft Weapon". There was an animated Star Trek episode based on The Soft Weapon, but I can't remember what I looked like, I just remember the producers weren't brave enough to animate an alien with two heads and three legs.
Other than those, I really liked the silly guns in Ratchet and Clank, epecially the Vacuum Cannon.
The zero G room gun from enders game looks pretty sick. I wasn't that much of a fan of the movie compared to the books but the gun design was pretty good. Also does star lords gun from marvel count? That's a pretty sick one too.