Which older multiplayer shooters are still well populated?

Looking into picking up an older multiplayer shooter. I think I'm stuck between Black Ops III and Battlefield 1 as both recently released "complete editions", but I'm open to other suggestions.

My main concern is making sure I pick a game with a healthy continuing player base. I likely won't play the campaign of whichever game I get, so this is strictly for multiplayer value.

Which older shooters are you still playing? Do you still find it easy to get matches in a variety of modes for the game you play most?