Rainbow Six Siege is the best and most tactical shooter I've ever played (I hear Squad beats it out, but can't say from experience).
On the other hand, Dying Light is the best zombie game ever made, and the free dev support it's recieved has made the second game a day 1 purchase for me.
Personally, I'd give Siege the win. I've never put that many hours into a multiplayer game, and I used to be a diehard Battlefield fan with 1000+ hours in just one entry. For a multiplayer FPS to unseat that, it'd have to be special.