I'll probably take the time to make some gifs later (I'm writing this in between school and band practice)...
In no particular order (for now):
Metal Gear Solid V: The Phantom Pain
Pretty much MGS5's Barrett M82 equivalent and it kicks some serious ass. I unlocked the customizing stuff as soon as the option was available and with a silencer and bigger mag added, this bad boy kills just about everything. Course it doesn't live up to its real-life counterpart as far as range is concerned, but this thing lets me snipe a gunship out of the air over some 400m, blow up tanks, it's just awesome. And few things are as satisfying as blowing heavy infantry - the bane of my existence, before - to bits with it.
This is probably a bit of an obscure one.... ages ago, teenage-me played a by now defunct online F2P shooter called "Contract Wars". It had the usual P2W going on, with real-world currency money buying premium coins that wiped the floor with the non-premium stuff, etc. And of course to get you addicted, they gave you small bits of premium currency when you started out. Now on one of my accounts, I saved all that stuff and bought a premium weapon - the OTs-33 Pernach. And that thing was awesome. Ammo somewhere in the neighbourhood of 30 rounds per mag and just my favourite sidearm ever. It just had it all - felt good, recoil-wise, pretty good stats, very useable iron sights and a horribly satisfying sound.
Out of all the incarnations of double barrel shotguns throughout several games, I think this was my favourite. Super loud, super fast ("Jakobs: fires as fast as you can pull the trigger"), almost instant reload speed. This was the one tier 1 non-premium gun you kept seeing people use, no matter their level. That thing was my default weapon for deathmatches and it may as well have been named Onekill-43 instead. I'm not a shotgun person, usually, but this one is the exception to prove the rule.
Garry's Mod or rather, TTT
Nuff said. Awful weapon for people who are too incompetend to use it (me), loud, recoil, headshots, boom.