I still can't work out why people like this game! I really really hated it.... when you compare it to the witcher 3 its just a really bad pc ported game.
I actually found TW pretty dull. Fantasy-by-numbers, buxom wenches who all want a bit of the anti-social, stoic loner. It just descened, generally, into fetch-quests. Furthermore, with combat featuring so heavily in the game, it was laughably bad.
Yes, it looked pretty and had boobies. But, apart form a couple of quests, it was a very standard affair.
Dark Souls is about learning and becoming fluent: you can always get smacked around by the easiest of enemies if you make an error. However, the feeling of achievement is unparalleled. TW lacks that. You level up and become unstoppable (which makes no sense in an overarching view, as you are supposed to be a machine at first). DS is different.
Personally, I think the atmosphere and complex lore is the icing. The overriding feeling of despair and futility, punctuated by small (and big) victories and into more sadness, of gods forgotten and battling once-proud heroes.
Nothing comes close. TW, in comparison, is an ARPG full of breasts.