I've been playing since 2011. It's a long term goal of mine to get a weapon from every class to Hale's Own. Most of my kills come from casual or community servers with minimal mods. So far, 6/9 of my classes have a Hale's own:

Scout: Scattergun
Soldier: Rocket Launcher
Demo: Iron Bomber, Stickybomb Launcher
Engineer: Widowmaker
Sniper: Sniper Rifle, Classic
Spy: Knife, Sapper

Pyro: Flamethrower
Heavy: Minigun
Medic: Kritzkrieg

Maining only a single class ensures you will only ever get to enjoy 1/9th of the game. Each class in TF2 offers a unique playstyle. To truly experience the game to its fullest, its my belief that you must gain a baseline level of proficiency with every class. While legitimately bringing a weapon to Hale's Own is only a crude indicator of proficiency with a class, I feel that its a decent gauge of experience.

Of the classes I currently have at Hale's Own, I would say Spy, by a decent margin.

Every other class has safe ways of going on high killstreaks. Soldier, Demo, Pyro, and Heavy can rush at the enemy team with a Medic strapped to their backs. Sniper can sit in the background and rack up kills without ever engaging anybody. Scout can pick his fights and run from situations that don't favor him. Engineer can place a sentry and gun down enemies busy fighting his gun.

Spy has uniquely wimpy combat abilities, and has to put himself in a vulnerable position if he wants to succeed. Even if you pop in and get a successful backstab, there is usually at least one experienced player ready to annihilate you the second you make yourself known. There are also a lot of situations where he can get outed and killed just by spam.

That is between Pyro and Demo, for different reasons.

Demo was easy just because of his raw power. He's difficult to play, but once you understand how to play him you can take on entire teams at once and win. He has the best crowd control of any class in the game, and is still reasonably strong in 1v1 situations. If he has have a kritz medic, he can spam stickies down a choke and rack up kills really fast.

Pyro, while still a WIP, is easy because he has the easiest combat of any class in the game. While he lacks the raw power of Demo, Soldier, or Heavy, he really can be reasonably effective just by W+M1ing. There are some tricks to master, such as airblasting and hitting flares, but in my experience these skills only make up a small fraction of his effectiveness.

The class I have most enjoyed grinding out was Engineer. With good shotgun aim, he can be a force to be reckoned with. He is the only class that can seamlessly transition between support, defense, and offense. He stands out as having the most control over the flow of the game, besides possibly medic.

If an engineer feels he needs more team support, he can place a teleporter and hang around it to stage a push from. If he feels his team keeps getting steamrolled, he can get a sentry and dispenser up to build a safe haven to build uber from. He's also incredibly underestimated from a combat perspective. Gunslinger + Widowmaker is a really strong combat combo.