It's difficult to rank a game that has lived as long as TF2. As much as the game has changed in the almost two decades since it release, it's core gameplay and community remain extremely close to what it always was. Although I don't welcome every change and update that has been made to the game, there is no doubt in my mind that TF2 remains one of the most outright fun games that have ever been created.
It's gameplay doesn't get old and it's charm still oozes out of every single corner of it's world and characters.

It is one of the greatest games ever made, and I doubt there will ever be anything like it again.