I've only played Demon's Souls, Dark Souls and Dark Souls 2. From those, Demon's Souls was a great mix of excellent gameplay and a subtle but very good story with two brilliant possible endings.
Dark Souls had a larger world and expanded the gameplay of Demon's in some ways, not all of them great, but it was overall good, gameplay-wise. The story however, I thought was terrible. Both endings were garbage and a complete let down after such a long game.
Dark Souls 2 took some elements from Demon's and gameplay-wise it was "alright" though I'd say the formula was getting really stale. The Valley of Defilement was pretty much copied from Demon's to Dark Souls 1 and then again in DS II? Come on now. The worst part however was the story. It took elements from Demon's but it didn't have nearly the same impact as that one. It felt repetitive, lazy and the endings were, again, shit.
I came off so disappointed after finishing Dark Souls 2 that I didn't even try the DLC parts even though I got the "complete" edition. I don't even know if I should try the other From Soft games when I get a PS4 since their recent way of making stories has been so subpar for me.
All that being said, I'm really intrigued by Nioh. I really like Feudal Japan as a setting so might give that game a shot at some point.