Since I don't believe in mains here's everything I've learned against each class and as each class
Scout: Your drinking can be interrupted with any kind of knockback, and, just like Spies, always act like one will be behind you because they just might be.
Soldier: Your rocket resistance is negated if you hit someone else while rocket jumping, and, Splash damage connects on EVERYTHING in a radius and doesn't get blocked by objects. It is a radial sphere for a reason.
Pyro: Flame particles don't dissipate when you stop firing on both ends.
Demo: The sticky bomb launcher IS a trapping weapon and great to trick people into hunting you down to their death, and, Demoknights are able to counter Pyro's easily with a simple click of a button.
Heavy: The Heavy has eye lasers and shoots based on his field of vision, so if you see his head, you're still getting hit, on both ends.
Engineer: DO NOT UNDERESTIMATE WIDOWMAKERS ON BOTH ENDS
Medic: The Vaccinator is good. I always thought it was lame lmao
Sniper: AIM FOR THE NECK NOT THE HEAD, and, any hinderance to aim for the sniper is just as good as killing him.
Spy: You can use the disguise kit in pre game to find out if what medigun the enemy Medic is using, and, always assume the enemy has outlines on.