KnitPick's Swish. It's SO soft.
Lorna's Laces Shepherd Sport and Shepherd Worsted are both superwash merino, as well as Karabella Aurora 8 (worsted), Aurora 4 (DK), and bulky. Joann's has a house-brand, Sensations, and their 100% merino is VERY soft, inexpensive, and also superwash. They don't market it as such, but I've run it through the washing machine enough times to know.