Anzula is a yarn company that makes perhaps THE softest sock yarn I have ever felt.
The base is called Anzula Squishy and the colorway I picked up is called Ducky.
At $34/ball, it isn't for every pair of socks you want to knit, of course. On the rare occasion you want to make something really REALLY special, I think this is a good contender.
My final vote for/against the sock yarn doesn't happen until I have knit up a pair of socks with it and worn them for a while to see how they hold up. Stay tuned for that. I'm toying with the idea of knitting up the Waffle Cream socks by Anne Hanson of Knit Spot.