The most important thing when choosing a sock knitting yarn is to think about what you’re going to use the socks for. Do you need them to be hard-wearing, everyday socks? Do you live somewhere warm or somewhere very very cold? Are you just wearing them to bed? These considerations will drive a lot of the decisions you make when choosing the best sock yarn for your needs.
Most sock knitting yarn is weight category two, often known as fingering weight yarn. The fabric you’ll knit with this is also comparatively lightweight, so it doesn’t add a ton of bulk. Most sock patterns are designed for this weight yarn. Some knitters also love knitting DK-weight or even worsted-weight socks. These socks are going to be bulky. There’s no getting around that. But if you live somewhere very cold, and you really need to keep your feet warm, the best sock yarn for you might just be a thicker yarn.
A challenge when choosing sock knitting yarn is that you need to pick a material that will absorb that sweat, pull it away from your feet, and let it evaporate quickly. This sweat wicking process will help keep your warm and dry. Wool is the best natural fiber for wicking sweat, with content being a close second. In this context, acrylic is the worst, since is has no absorbency or wicking capabilities.
Saying that gauge doesn’t matter in accessories, or that if the maker doesn’t get gauge to ‘just go up/down a size’ can be a recipe for disaster. Sometimes this approach seems like it can work, but what we think of as working is actually the maker compromising their expectations and settling.
Think about the theory behind how ropes are constructed. The more strands in the rope, the stronger the entire rope will be. The same theory applies to sock yarn. That means that generally speaking, a single ply yarn is not going to be the best yarn for knitting socks. It feels lovely in the hand, but it won’t stand up to the wear and tear. Most sock knitting yarns have two plies, or sometimes three.