Definitely TF2 and the infamous "hats". Other games may have sold cosmetics, but none truly gamified it like Valve. They even had a proper economist [0] whose whole job was to tweak and optimize the system to make as much money as possible. In 2011, they switched TF2 from paid to F2P, which also was a huge step forward for the microtransaction model. CS:GO soon followed suit.

[0] https://www.businessinsider.com/yanis-varoufakis-valve-game-...