I would say that Heavy and Pyro have the best cosmetics
The reason?
Well since I'm usually a Friendlie I like cute hats, as we all know Heavy and Pyro have the cutest cosmetics.
Pyro - Trickster's Turnout Gear | Area 451 | Flammable Favor.
Heavy - The Gabe's Glasses | All-Father | Officer's Ushanka
In short Heavy and Pyro, but I like those cosmetic load outs specifically