The sniper is by definition OP. He can aim freehand, then tap right click to insta-scope for an instant kill, as the moment the scope is activated crit headshots are possible. There are no ballistics, unless using the huntsman; you click you hit. He has an insta-kill from any distance and the layer of "Skill" is the ability to move your mouse. The only class that can stand up to a sniper is another sniper or a spy. I've listened to the developer commentary and they sure as hell haven't adhered to their own words; they wanted it to need skill, while also maintaining balance. That said what they SHOULD have done is made the base damage low even scoped. You would need a high charge to headshot, not "any headshot insta-crits"
Thats 2 classes out of 9 that can counter it from over about 200 meters which is typical engagement distance. Everything else gets obliterated by the time they get a shot off due to weapon spread of other classes, and that's why some people, particularly edgy cringy teen MLG pro trash talking keyboard warrior scumbags think its the most skilled class.
Everything else is effectively balanced far as I'm concerned, even pyro.