Left for Dead. Back 4 Blood is a poor imitation of L4D, and there's no real reason to play the former over the latter. B4B was just flavor of the month and rode off the coat-tail of a game by people behind L4D.
Going by Steam data. There are still people playing L4D 2 to this day, which is more than can be said about B4B - which released a couple months ago.







