Dark Souls 1 by a reasonable margin, in my opinion. Although I think Dark Souls 1 falls pretty far from the "always completely fair and perfect" mantra some fans throw out constantly, it's a really good, well-designed game. The Artorias fight is the most well crafted part of the entire Souls series and highlights the best of the game.
Dark Souls 2 was plagued by a number of things that just made the game feel much cheaper and annoying compared with 1. Souls 2 had wonky hitboxes, broken enemies, and replaced quality and quantity. For example, the Artorias fight, if designed by the way of Souls 2, would have been a clunkier fight with Artorias flanked by a half a dozen minor enemies. Souls is at its best in well-designed one on one fights, not shit like the Royal Rat Authority. Souls 2 just throws a bunch of enemies at you and assumes that's just as good. The vaccuum hippos and infinite stamina mace drakekeeper knights are just shitty game design.