The activity is varied depending on which platform you're playing. While I only play PC, I'm certain all Multiplayer games are still active, however it also depends at the time of day you're playing. But if you want to play the most active multiplayer, that would be AC4. Although choose whichever one you like, but from my personal experience all the Multiplayer games are still active. The least active however is Brotherhood, and you'll need a lot of friends to help you, and ACR is a little inactive but there will most likely be a session here and there