Plymouth Superwash Merino
This 100% extrafine merino yarn (extrafine meaning that it is in the 19 micron range instead of the coarser and more typical 22.5 micron range of most merino!) is machine washable and comes in a lovely array of colors. Most customers comment on its softness and springiness.
Recommended knit at 5 sts/inch on a US 7; some customers knit it at 4.5 sts/inch on a US 8. Each 100 gm skein has 218 yards.
If you don't require machine washability, the Cascade 220 is a similar gauge worsted yarn and also comes in a lot of great colors and offers a great value. And if you seek a hand-dyed/variegated yarn that knits at this same gauge, be sure to check out the Rios by Malabrigo.