In all honesty, none of the subclasses are useful. Generally speaking, no subclass can do a job that another class can't do better. Nintey nine percent of the time you'd be better off just playing a class that specialises in a specific role rather than a weird amalgamation of a playstyle that doesn't fit the class itself *ie. shotgun heavy, battle medic, troldier, and even in that rare one percent, it's only a slight advantage. As a whole, all subclasses just kinda suck.
The exeption, being demoknight.

I'll be the first to admit, like all subclasses, demoknight is situational. The map, team composition for both teams, and weapon choise dictate whether or not demoknight will even be slightly useful, and if even one of these variables is off, demoknight becomes little more than a wast of attention and health kits, and this is true of pretty much every class and/or weapon in TF2. You need to change what you're doing if it conflicts with what your team needs. It's Team Fortress, emphasis on TEAM.
So, in what situation is demoknight useful, and what makes him different from the other subclasses, allowing him to be more commonly useful? Simple, he has three attributes no other class can have at the same time.
1:He's tanky
2:He has a good movement speed
3:He can pick off important targets and not die immediatly afterwards
This makes demoknight good at somthing no other class could truly be said to be good at, making crucial picks and being pocketed simultaniously. His sheilds let him tank, his sword lets him pick, and his speed lets him get out alive and do it again. Add a medic to this equation, and suddenly you have a spy who can pop out of nowhere with 300 hp, run through your whole team, kill your med, and be complaing about having to clean idiot off of his shoes from the other side of the map ten seconds later.

As demoknight you're only actually useful if you're doing what only a demoknight can. You should have two main objectives as demoknight:
1: Protect teamates
You have high movement speed and stupid range, you're gonna need to use this to help out your teamates. Scouts and spies are basically just lunch for a demoknight, and thanks to your speed and ability to charge out of bad situations it's best if you keep moving from place to place, checking for snipers, sentries, stickies etc. Obviously it's only really useful if you tell your team, so stop being weird and use voicechat, or at least type it in teamchat. Your team might just decide to useful for once, likely because they feel bad for you.
2:Pick
Medics, heavies, soldiers, demos, you can easily pick off most classes with a well timed charge. Even pyros and scouts can be easy kills if they don't see you coming. You put out stupid burst damage, and with a team for backup you can suddenly pop out of the background and kill damaged classes without much risk or effort. While medics always take priority, damaged heavies shouldn't be ignored, just make sure they don't see you coming. The one thing I'd say you need to do is think. Learn to judge scenarios to decide whether or not a kill is worth it. While throwing your life away is not recomended, often times charging in and taking out a heavy or a medic can win games, so don't think of yourself as too important.

Alright, time for the less interesting than expexted part. When it comes to demoknight loadouts they should either assist in killing, or staying alive after the kill. Let's start off by talking about the best sword for picking.

The Claidheamh Mòr
The Claidheamh Mòr is more useful than it gets credit for. That bonus charge distance is essential at getting some picks, and giving demoknights even more space to work with is, dare I say, nearly overpowerd. Luckily you'll die after every kill because of the damage vunerability, so you basically just kill the medic and then die. Luckily, unlike the Scotsman's Skullcutter, you can still function as a demoknight even when you're not aiming for a sick frag. Just be a bit more careful.
And as far as tanking:

The Eyelander
When you have four heads you're nearly unstoppable. While getting to that point is a pain, once you're there, you've become a serious threat even without your charge. Getting in and out of fights is a cakewalk with your faster-than-scout speed, and with your health you can charge though teams to chase down kills if your charge doesn't do the trick. Just don't get too greedy. Few things feel worse than having to get back up to four heads after a stupid death.

While those two swords are the best at those two specific jobs, most other swords have some simple use to play around with, and they generally don't come with such a crippling dowside attached.

The Persian Persuader
It gives you charge back for hits, and let's you use ammo kits to get charge back. It's my personnal favorite, as it allows for a more spy-like hit and run playstyle. Just don't take ammo from classes that really need it.




The Half-Zatoichi
Living fights couldn't be easier with this thing, just make sure you actually get the kill. Plus, it allows you to bounce from enemy to enemy, gaining health along the way. Letting you give yourself overheal is a nice touch too.





The Scotman's Skullcutter
More damage means easy picks, and it lets you hit some important damage thresholds as well. But, here's the real question, why not just play spy if you want to only be good at picking, die right after a kill, and take forever to even get into position for said kill.

I'm too tired to bother with some sort of ending that wraps everything up. Goodnight. Have a meme.