I'll agree with Jess about the baby alpaca. I'm knitting some up now and it is so ultra soft. And touching the wool hat I'm working on right after the alpaca, I can feel how rough the wool is (even though it feels soft when I'm working with it).
Two others I felt at my LYS yesterday that were super soft were silk and bamboo. I believe bamboo is washable, although I'm not positive.
Some organic cottons are also super soft, since they haven't been so highly processed. Of course, they will shrink, so you'd have to allow for that.