Combo Pyro for starters
If we talk about the best classes for high damage output then pyro is certainly one of the best classes. Pyro deals constant damage and also deals damage when he is not attacking. Pyro has an afterburn on most of his flamethrowers and secondary weapons. Pyro is already a very good class for dealing with a lot of damage in a short time. To support this class, the community has made a subclass called the combo pyro. Combo pyro is a very popular subclass in the team fortress 2 community. Most competitive players and content creators follow this kind of playstyle. This has helped this playstyle to be more famous. This subclass may sound very simple but it is not something that new players should be trying instantly. This requires precision and fast reflexes. Playing this subclass requires you to build muscle memory. If you don't know what combo pyro is then let me explain. This subclass switches weapons mid-fights to deal more damage. For example, I am using the stock flamethrower and the flare gun. The stock flamethrower is just a normal flamethrower while the flare gun deals more damage to enemies that are already on fire. So, if I use the flamethrower to light the enemy on fire then instantly switch to my flare gun to deal critical damage and finish off my enemies then I am playing as a combo pyro. Maybe it sounded weird but that's combo pyro in a nutshell.
We've talked about what a combo pyro is but now let's talk about common loadouts that suit the playstyle of combo pyro. The most common loadout for a combo pyro is degreaser as primary, flare gun as secondary, and axtingusiher as his melee. This loadout is very common. Players using this loadout will burn their enemies with the degreaser and then deal quick 90 damage with the flare gun because the flare gun guarantees critical hits on burning enemies. Sometimes they will use the axtingusiher to finish off their enemies. Another popular loadout is dragon's fury as the primary, flare gun as their secondary and power jack as their melee weapon. They get close to enemies using the power jack and then bully their enemies with the dragon's fury + flare gun combo. This last loadout is also used by hybrid combo-pyros. Hybrid combo-pyros are those players who combine other playstyles with the combo-pyro subclass. Their loadout is mostly composed like this, with dragons' fury as their primary, panic attack as their secondary, and power jack as their melee. These players use the power jack to get closer to enemies and then burn them with the dragon's fury. After burning them they will use the panic attack shotgun to finish them off. These were the most common combo pyro loadouts in team fortress 2.
Once you choose your loadout of choice then try practising it. Use this technique to kill the bots in the tr_walkway_rc2 map. When you close the gap between you and your enemies then you should burn them and airblast them at the same time. When you airblast them they will be paralyzed for some time. In this small window of time, you need to instantly swap to your secondary or melee and damage the enemy as fast as you can. If you use the flare gun or the axtinguisher to finish them off then you can deal much more damage to your enemies. Being a combo pyro, you don't have many options against a crowd of enemies. If you are stuck in such a situation then try to mix up your playstyles. Since you are a combo pyro you won't have weapons like phlogistinator. Because of this, you won't be viable against a group of enemies. You should also prioritize slower targets. Using the combo pyros technique on scout won't work most of the time because they are very fast. Combo pyro is a subclass whose basics are pretty easy to learn but very hard to master. Switching to your other weapon and finishing enemies in milliseconds may sound easy but it is very hard. Well, if you have picked up this playstyle then I wish you the best of luck. Thank you and have a very good day.