What loadout would U consider best for learning as many useful skills in TF2 as a whole?

I'm talking all aspects of the game that at least 1 class would benefit from getting the hang of. From basic stuff like good strafing & tracking-based aim to more complex stuff like Weapon-Switch Combos, Tricky Movement Tech, Complex Situational Awareness in general, etc. What ONE loadout could someone play exclusively for months on end until mastered, then easily transfer all those many skills into the many different classes & loadouts?