Fuck. Too many choices bru.

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.

But there's so many more!