It all depends on which you’ve played first as once you play one the others become significantly easier. I went with bloodborne as the hardest in that list due to the drastic change in gameplay style or more that you’re stuck playing rushdown instead of playing with other types of builds.
BB itself isn’t all that too hard anyway until you get to the last boss of the old hunters DLC but he too can be beaten easily with enough patience.
Demon Souls and DS1 are really only difficult due to their clunkiness but are really exploitable. Great games overall though.
DS2 has questionable hit boxes and some bullshit here and there but it’s pretty easy overall aside from the DLCs where it finally ramps up in quality (except for all their little gauntlets which are just flat out annoying)
DS3 is the easiest of the bunch and most fair unless you’re playing online.
You forgot Sekiro btw, it may not be full on souls but it’s still up there and I would say it’s the most difficult of the bunch.