Undeniably the most overall influential, original, unrestricted, passionate, dedicated, incredible game to ever be made.

TF2 shaped internet culture for well over a decade and continues to do so.
TF2 came to be during an era when games were trying to be everything TF2 wasn't. Yet it committed to it, evolved, grew, its identity remained the same and it alone still stands strong.
TF2 as a PvP FPS had and still has an extraordinary amount of modability. Furthermore, there is no filter on what you can say or do.
TF2's process of development is nothing short of fascinating. From designing in black & white to ensure readability, the depth and character behind the classes and their voices, to the small things like what you see in the flavor text. You cannot play this game without seeing firsthand the overwhelming amount of passion put into it.
TF2 has proven to have some of the most dedicated fans, voice actors, and for as much flak as they get; developers--out there. Additionally, community-run servers and gamemodes are still thriving.
TF2, while not the undisputed champion in any of these categories, is unmatched in sheer quantity and quality with what it achieves.

No lip service can do it justice. There will likely never be another game like it, nor will there ever be another game as good as it. Its status is legendary--enjoyed equally by those who have or will go onto change history and by those whom nobody will remember.

Enjoy it while it lasts, be happy you got to be a part of the experience. One day, it will be gone, and when that day comes, be thankful it happened, not sad that it's over.