For anybody over the age of like, 30 who played PC growing up, TF2 was the team shooter, even more than CounterStrike, mostly because it was free to get started in and had a low skill for entry, lots of collectables/outfits and a high skill ceiling.

I would guess that was still mostly true up until maybe Overwatch. I know TF2 is still very popular with large sections of online gaming groups.