It Has a Really Good PvP Scene
This one is a little debatable, as From Software has been making little tweaks and improvements to the PvP and co-op elements with each entry in the series. Most will argue that Dark Souls II was the best for PvP because of the sheer number of character builds you could play around with, but the player base has severely dwindled since. On the other hand, Dark Souls III still enjoys a decent player base, though the lack of PvP covenants, among other issues like host latency and being unable to punish Estus spamming, hold it back from being the best.
Though the game is pretty aged at this point, Dark Souls Remastered still offers some of the best PvP action you could ask for in a Souls game. There’s no power stancing (read: dual-wielding same weapons) in this game, but the build variety is still pretty solid. There are good PvP covenants, and now that invaders and co-op partners can actually drink Estus, this makes the PvP aspect even more competitive and exciting.