Softest diaper I ever wore was the Molicare Super Plus, the purple ones before they were discontinued. Blew my mind at how soft they were, inside and outer.
Softest booster, now a revised product sadly, was the Abri-let Maxi before the revision. Really thick and poofy when dry and did an amazing job at wicking.
Rears diapers are the softest imo with ABU in close second.
Can't say which boosters are softest, haven't tried any more than ABU, Northshore and the revised Abri-let maxi boosters. None really feel super plushy soft to me.