Here's a list of weapons I find overpowered by class. Though not everyone would agree, I find they detract from the game on the whole:
Keep in mind I may be a bit biased as I only really play competitively:
Soda Popper - If you type +reload in console, it will increase reload speed, which I feel makes it too strong. Most high level scouts don't abuse it though, as they don't want to throw off their 6s game where this is banned.
Baby Face's Blaster - It is way too easy to get your speed charged up, the downside is almost negligible if you just get a few chipshots in, the speed you gain from this can cause hitscan registry issues, and you're just generally untouchable by most projectiles. Incredibly broken, one of the most overpowered, definitely the scouts most powerful weapon.
Crit-a-cola - While it has a heavy downside, it allows you to drink, turn a corner, and instantly kill anything close by. They can do nothing to prevent it as you're moving faster, you're hitscan, and you have the mobility of double jumps while its up.
Mad Milk - Balanced in most modes where there are many players, but the fewer players there are, the stronger this becomes. It will completely turn small skirmishes in the milk-user's favor, and if used on the way in to engagements is incredibly powerful. My go-to choice in highlander.
Pocket pistol - I feel this weapon is almost balanced, but with the lack of pyro presence in 6v6, it gets a ban as it's almost a direct upgrade. A scout with 210 on a full buff is a freaking monster.
Flying Guillotine - Even without sandman synergy, you can just toss this high up in the direction of your enemy every 6 seconds, and occasionally it will mini-crit, doing a ton of damage, and if working with your team, can just provide huge openings.
Sandman - See explanation for guillotine, but with a stun instead of damage+bleed.
Atomizer - Melee is not a kill you even care about as scout anyway, and you will definitely save more than 10 hp by dodging the damage source. It allows scouts to completely dominate soldiers, even buffed soldiers. Incredibly powerful.
Beggar's Bazooka - Allows the ability to turn a corner and shoot three rockets at unsuspecting enemies. Allows impossibly hard to dodge angles of engagement, and allows the soldier to fire rockets endlessly without the need to reload as the stock would. Weaker with long range spam, but with the mobility it grants, who needs them.
Reserve Shooter - Incredibly powerful. Allows you to instantly kill anything you shot in the feet with a rocket. Dominates scouts completely, and is just a very large source of damage. The drawback is negligible, 4 shots and 4 rockets is still plenty of loaded damage.
Disciplinary Action - While I don't find it overpowered in itself, it removes the downsides of other classes. It allows you to get snipers, pyros and heavies to mid-fights in 6v6 and is just generally a detriment to the game.
Degreaser - I find it very overpowered. However it is also what enables pyro to be effective in the game. So while overpowered, I find it enables the class, and does not detract from the game.
Reserve Shooter - See above. Made all the better by a pyro's playstyle and ability to push any class not only in the air, but into a predictable arc to get full meatshots with ease.
Loch-n-Load - The grenades do insane damage. With 3 loaded, and the ease to hit them, it just leads to incredible damage output from any range. High level demomen were getting logs where they did more damage with LnL than stickies.
Stickybomb Launcher - Stickies are straight up OP. Well aimed do incredible damage, deny area, 8 loaded at once gives high potential. While not as strong as they used to be, are still incredibly strong. HL is based around the damage output of this weapon, and 6s restricts one demo maximum due to it.
Ullapool Caber - Way too easy to just bomb in and get easy kills. It's like a market gardener that you only need to hit the person at any point, rather than while airborne. It does have a significant downside, however with teammates around you likely don't need to finish people up with melee anyway. You would have more scenarios where caber was a boon than a detriment in play.
Natascha - While I don't find it to be overpowered, it definitely isn't fun. A heavy using it is vulnerable, and it doesn't do as much damage, however a defensive heavy just tagging people on lasts (think of trying to push in granary last) isn't fun or interesting, and detracts from the game.
Sandvich - Medium health pack for allies on demand, is incredibly strong. It allows aggressive posturing with much less risk, as you can just heal up your medic instead of backing up to get him health.
GRU - While I don't find them to be too bad outside of 6v6, it encourages running a full-time heavy in 6v6 which cripples the pace of the game and is not fun. I find it to detract from the game. In a slower game-mode where heavy is already present though (HL) isn't as much of an impact.
Widowmaker - In the hands of the skilled player is infinite ammunition, and even metal gain.
Pomson - While not overpowered in itself, the fact that it can be spammed to reduce uber of enemy medics it is a massive detriment to the game. You have to player slower, further back, more hidden just to make sure you don't lose uber to these shots.
Wrangler - It gives the sentry such a high effective HP that it's completely ridiculous. It would take 6 stickies to bring down a wrangled sentry, and with the ability to aim your sentry, you can use sentry rockets to push stickies away. It is handlable with proper teamwork (spy sap, coordinated spam, sniper fullcharge, etc) but considering it took no effort on the engi's part to press 2, it is incredibly strong. It slows the pace of the game massively, and I find it to be a detriment to the game.
Short Circuit - I feel it is too easy for this weapon to do its job. You don't need to time it like you do an airblast, n'or do you need to aim it. It completely nullifies several classes trying to push in to your sentry and is just not fun.
Gunslinger - I feel the area denial the mini-sentry applies comes too easily, or at least too cheap a cost. It requires the presence of a longer range class that can deal with it, with very little effort on behalf of the engineer. Even when dealt with, can be replaced. It's not the sentry's ability to kill that makes this too strong, it's the area denial.
Quick-fix - Quick-fix + Heavy = Stalemates for years
Vita-saw - The advantage provides is not something necessarily overpowered, but the advantage it provides requires both medics to be running it. The medic using it essentially spawns with a 20% uber advantage in many scenarios throughout a game. It's incredibly huge to have that kind of advantage over the other player.
Jarate - See mad milk.
Darwin's Danger Shield - The hp is nice, however the important thing is that you are now unkillable to a sniper's headshot or full charge bodyshot while at full hp. This is increeeedibly huge. In HL, snipers play close to medic to get buffed which accomplishes much the same thing, however this would let you do it regardless of positioning. Also, see vita-saw.
Enforcer - This weapon is incredibly strong. It allows you to 3-shot light-medium classes, and 4 shot many others. In the hands of something with solid aim, they can do tons of damage with this weapon.
Red-tape Recorder - Is all but guaranteed to get one downgrade, as you have to deal with the spy before worrying about unsapping. If you unsap first, the spy just kills you. Unless you have someone with you watching your sentry at all times, you're likely to downgrade. It also accomplishes the main point of the sapper (disable, wrangler turns off) in a team environment as well.
Well that was a ton of effort... I feel like I got more whiny as it went on as well, sorry if it seems more subjective as it goes on.