CoD has never really done too well on PC.

I think MW2 has a few active thousand on it, maybe a little lower now; however, a lot of lobbies are hacked.

Black Ops 1 had like 500 players on the last time I played 2 months back.

MW3 had like 2k-ish on when I last played, but TDM was the only game mode really alive.

Black Ops 2 had different numbers all the time. NukeTown 24/7 is like the only gamemode active, with a few Domination and TDM on other maps.

Ghosts, died out quickly due to numerous problems.

Advanced Warfare was completely dead, less than 100 players on and I had to refund the game since I couldn't play.

Black Ops 3 is the most active with probably about 4-5k players. TDM and Domination are the most active servers.

IW, not too sure about as I played that a few times and that was it.

Modern Warfare Remasterd started off great, but slowly died down after the numerous problems with the PC port and because of adding in Supply Drops, new weapons, ect; when it was claimed to be like CoD4.

CoD 4 and WaW have an active playerbase, but that's mostly for like custom game modes or modded lobbies. Very rarely have I found a legit lobby, so who knows how they will fair for you.

Would have to say Modern Warfare 2, as it was the only decent Call Of Duty period.

I played modern warfare 2 a long time ago and it had around 7k people playing it.

black ops 2

You could try the recent Modern Warfare remastered if you want multiplayer, but this means buying infinite warfare as well.

Based on the picture provided, BO3 is the most played COD on PC at the moment.
[ Register or Signin to view external links. ]