What an interesting topic ! I never thought about that before, and it's quite a pleasure to come back here to such a discussion. Here are my two cents :
-AA12 : arguably the original "Zed Nuker", both in KF1 when we didn't really have a T3 and in KF2 when the original four perks lacked firepower... safe for the Support of course ! It's good against everything, it has a simple but iconic look and the Fostinator himself praises the gun whenever he can. It's a quintessential shotgun, just like the...
-Boomstick : remember when it used to weight TEN kg? And when you couldn't reload if you still had a shell? How times have changed, and yet the Boomstick always been one of the strongest T2 in the game. You could survive the entire game with just that gun... An economic marvel. Only its total ammo can lead you to some troubles in later waves, as it's fairly low. Otherwise... It's just perfect, and just as iconic. Especially considering most shooters have very poor double-barreled shotguns (for obvious reasons... Only two shells !)
-M32 : For someone who was so AGAINST its inclusion in KF2... I use that gun a lot. I was afraid of its high amount of power, and the annoyance of its smoke clouds for the rest of the team... But boy, how could you hate that grenade launcher? It's great to completely delete a Fleshpound, or to bombard a legion of zeds. It looks super-duper cool too, and feels just satisfying to use. Although I don't like the M79 that much anymore... I would include it as well as essentials to the Demolition perk (although, as one of my favourite perk, I could say so for nearly all his guns... RPG-7, HX-25, Kaboomstick? All classics)
-Trenchgun : Similarly, I LOVE the Firebug perk. Already did in KF1 and absolutely does in KF2. I think all of his shared weapons are classics by now, but the Trenchgun is my absolute favorite. It's powerful, it's cheap, it's reliable... And just like the Boomstick, you can use it well into the latter waves. It's surprisingly effective at range too, and with the Zed Shrapnel skill, it becomes ridiculously fun ! Definitely a must-have, but so would be the Flamethrower (it's a pyromaniac perk after all). The Mac-10, albeit fairly disappointing in KF2, also holds a place close to my heart... It was the first weapon added to the perk's arsenal after a long time of using the flamethrower only. It was the most precise gun at his disposal, the main answer to Husks and overall a quite clever idea.
-LAR : Let's complete the "Cheap Trilogy" with the lever-action rifle. Far less useful in KF2 compared to its ubiquity in KF1, it is still a very impressive gun for its tier. Accurate, decent mag, reloads that you can interrupt, and an absolutely delightful design. What I miss more is probably how often it was seen in the hands of a Field Medic or Demolitions, up until they got their own guns. Even the Zerk often used it to deal with ranged opponents ! The modest LAR kickstarted more games that I can remember. It deserves to appear in every KF game.
-Deagles : It should be called "Handcannon" forever and ever, even with the inclusion of the .500. There's a reason why it appeared in the Matrix and other popular medias shortly after its creation : it's an absolute icon. Pure raw force packed in a shiny metal piece of art. Similarly to the LAR, it was often seen in the hands of other perks... And was actually my main sidearm in KF1, and the gun I used the most for the Gunslinger. A single Deagle is fairly cheap and can help you fend off most monsters up until the FPs starts to show up. With the humble 9mm, I believe it's the only pistol really deserving to appear each time.
-Crossbow : It's almost insulting how little I use that weapon in KF2 compared to the absolute killing machine it was in KF1. I don't think I ever saw anyone using the M99 in KF1, safe for new players. You were either a Xbow or a M14 EBR player (the latter being also absolute trash in KF2). And for good reasons : the crossbow could make short work of anything. And even as a single-shot weapon, you reloaded so fast that nothing could come close to you (and it they did : you had your handcannon !). How many games let you snipe with a crossbow anyway? We need to have it back !
And I believe that's it really. I like a lot of other guns, but I feel like a lot of them could be easily replaced. Commando's armory for example could functionally get replaced by a plethora of other assault rifles.
I also believe we should reevaluate every perks arsenal in a future game. We don't need THAT MANY guns. It would be better to have fewer options but make them more distinct... And all useful in their own way. But that's another debate we already had numerous times.
