Are we talking about a soft outer layer, a soft interior or all around softness?
The softest interior probably a tie between ABU brands and Bambinos with Rearz brands in a close third. I also recall the Crinklz that I had being very soft too, but I found my experience with them to be a little forgettable, they aren't a go to of mine. I have yet to try Tykables but I heard the new line is very cozy.
Softest outer layer, I'm gonna have to agree with Argent here, I find the exteriors of the new SDK's to be incredibly soft and smooth, they really are like wearing a pillow if you fluff em and stuff em before hand, those diapers are also incredibly soft and squishy when wet. I have heard that the new Tykables have a very soft outer layer as well, hence why I intend to buy some as soon as they're available in Canada (should be the end of the month as one of my Canadian AB suppliers told me).
For overall softness, I'd go with the ABU brands and Bambinos. I like Rearz for their performance and thickness, but they are far from being the absolute comfiest diaper on the market, the interior is nice but the outer layer is rather coarse, almost like a shower curtain. Crinklz are OK too though and I've heard great things about Tykables.