Dark Souls 1 has the best world/level design
Dark Souls 2 has the best build variety
Dark Souls 3 has the best bosses I would say (though that’s a bit more subjective than the other two points I made).
One big thing DaS3 lost from 2 was the Dual wielding system. In DaS2 you could mix and match any weapon of a similar type and use a stance to do special dual wielding attacks. In DaS3 there are weapons you can do special dual wielding attacks with, but they’re only select weapons that come as a set, so there’s less experimentation you can do with it. From what I’ve seen though, Elden Ring will bring back DaS2’s mix and match system. It’ll also bring back double sided blades that previously only appeared in DaS2 as well.
In terms of build variety it seems like it may be a successor to DaS2, so that has me excited.