the best souls game?

Dark Souls 1 best Dark Souls.

What about Demon Souls though?

Bloodborne. It only tires you with one game worth of overbloated lore that largely goes absolutely nowhere, instead of 3+1 games worth of the mostly same stuff. Otherwise, I am still yet to find a single difference in the games beyond mildly changing the order of the level and boss archetypes.

Nioh

DS1's ambientation was flawless. Too bad that the gameplay itself was really clunky. DS2's forte would be the major improvements in gameplay, and its DLCs, but it's so green and orange that you can hardly call it a sequel to the first one. And then we have DS3, which is incredibly linear, but has gorgeous graphics, an outstanding ambientation, a well polished combat system, and excellent endings for a souls game. I'd stick to DS3 only because of the huge fanservice in the game, and the hours of pvp I put into ite.

I would vote for Dark Souls 1 but that version "prepare to die" has SO many bugs and you have to actually use a patch that the comunity made (if you want to play it on steam) to work, so... If you are talking in the console, the first dark souls for sure. But on pc I prefer dark souls 2
(but on top of all, bloodborne is still better)

If you're too timid about the difficulty then you want 3. If you want the most authentic version get 1.