Scout : the fish
I like the fish. The fish is good with crit a cola. I once got a kill using this combo
Soldier: beggars bazooka.
It's, like, 3 rockets really fast. Also, if you just hold the button for a little bit, you can fire a single rocket very fast. In a way, you can spam out rockets faster than with stock, because of the weird reload
Pyro: the jetpack
It makes you fly. People will be like "I'm all safe on this high ground. Absolutely no chance a Pyro will fly up here" and then Bam! You fly up there and they're like "ohh nooo!" and then they're dead
Demoman: the pan.
Like, I know it's just a meme that this weapon always crits, but for real, melee crits are insane. If you do like 800 damage over the last 20 seconds, your crit chance is at 60%. 60%! Your chance is bigger to crit than not to crit. Playing demoknight, with the splendid screen, doing two charge + melee crits brings your crit chance up to 49%. So, like, if you play your cards right, the pan is like a budget KGB for demoknighting
Heavy: the buffalo steak sandvich.
Choosing an underrated weapon for the heavy is hard. He doesn't have much variety. So here it is, the buffalo steak sandvich. Now, you might be thinking, does this piece of garbage have any redeeming qualities whatsoever? Let me tell you about the heavy and speed, and in fact, why the buffalo steak sandvich is banned in comp. It increases heavies walking speed by 30%. That means he gets to the front lines from spawn 30% faster! Does it stack with the gloves of running urgently? No, it does not, but, like, if you wanna hold a sign up while running from spawn, now you can!
Engineer: the gunslinger.
In many ways, minisentries are just a pure downgrade from regular sentries. But this line of thinking ignores the rest of engineers kit! Using the gunslinger, you are free to focus a lot more on your other buildings. Don't underestimate the power of a teleporter or dispenser. Using the gunslinger, you've got enough metal to build a sentry and a dispenser, increasing your potential setup time. It's also really cheaky to hide minisentries in places nobody looks. Sure, you can hide any sentry in a corner nobody looks, but a minisentry is so much less costly when the enemy eventually catches on and destroys it
Medic: absolutely nothing.
When playing medic, just run the stock medigun or kritzkrieg, the crusaders crossbow and the ubersaw. Nothing else really compares to this perfect loadout. I guess the vaccinator can have its uses, but it doesn't turn the tide of battle like an uber or crits does
Sniper: the huntsman
No aim? No problem! Just spam this in the general direction of the enemy team at head high and you'll definitely get a kill! Maybe? Hopefully? Hey, they call it the lucksman for a reason. It also has pretty much the best taunt kill in the game, apart from arguably the heavies pow (due to range) and the escape plan, when exploiting it under bridges and such
Spy: black rose
I might be cheating a bit here, but spy is so hard to choose an underrated weapon for. He has 3 watches, each with around a third of pick rate. All of his knives are kind of balanced, I wouldn't say any of them are underrated. His guns... I dunno, I just use the letranger. Sappers... I mean, come on, the RTR is only better in like 3 situations. But there is one weapon that's so forgotten, that I'm just gonna call it underrated