I must say that the one yarn I have used that I just loved working with and petting was Debbie Bliss 100% silk. I was always running my fingers over my knitting when I wasn't even knitting! So soft and silky .......mmmmmm yummy
the softest i've knit with (not just touched) would be Misti Alpaca, or alpaca in general. "Poor man's cashmere" i'm told...
I have fondled Malagrigo, and I agree it's soft, but I can't get past the vinegar smell, it can't be good when damp...not knocking it though... just saying "hmmm"
In addition to silkiness, fluffiness, and hi loft, I would add a critical factor in yarn is it's elasticity. It is what gives wool (and aryllic) ribs their extreme "body" and stretchiness. My softest wools/fibers aren't necessarily the best for ribs. But you can exploit that factor too. I knit the swatch on my bloghttp://angora-rabbits.blogspot.com/ out of cotton first, and then angora, because angora has little elasticity and would block out to show the rib pattern well, instead of scrunching in and forming a rib with a lot of body. This stitch isn't striking at all on regular elastic wool. So, there is a lot of drape to this sample. But something almost as soft in pure wool would have almost no drape, even at the same guage.