There is no “best”. It all depends on what the pattern designer wants to achieve.
Certain traditional techniques require fairly specific qualities of yarn:
- Aran sweaters require a round woollen-spun Aran-weight pure wool yarn, ideally in the grease.
- Icelandic sweaters (lopapeysa) require slightly hairy, unspun lopi wool from Icelandic (breed) sheep.
- Fair Isle knits require grabby fingering weight yarn from Shetland (breed) sheep.
- British fisher ganseys need to be tightly worked in worsted-spun sport-weight pure wool.
- Cowichan sweaters should be worked in greasy handspun bulky-weight wool in natural colours.
and so on. You can, absolutely, knit an Aran sweater in bulky-weight cotton, but prepare to be deeply disappointed in how cold it is, and the way it expands with every wash. A Cowichan in qiviut would be toasty-warm, but outside pretty much anyone’s budget. And a Fair Isle jumper in acrylic yarn just doesn’t bear thinking about, especially if you’re going to knit it properly with steeks.
For other knit items, the appropriate yarn is the yarn that does what the designer wants. Linens and silk, and the higher-end breeds of sheep, produce a great draping effect. Alpaca and cashmere are luxuriously warm, with widely differing price points. Woollen- and worsted-spun yarns have differing uses, which can vary depending on the source fibre – worsted-spun in, say, Wensleydale, makes marvellous – and sexy – thermal underwear, but in mohair is perfect for all-weather overcoats, while woollen-spun yarns trap heat and are usually terrific for textured knits.
Of course, not everyone is going to have access to, or be able to afford the yarn recommended in the pattern. Many designers get free yarn as part of the deal when knitting for a yarn producer, and yarn producers offer such freebies as part of their marketing campaigns to get knitters to buy their yarn. Learning to substitute yarns properly is an essential skill for knitters, at least as important as learning different cast-ons and bind-offs (sturdy, firm, relaxed, decorative, etc.).
Make a policy of learning about different yarns and their qualities, what they’re best suited for, and – most importantly – what YOU like about them. If you’re a vegan or allergic to lanolin, wool isn’t your best choice; similarly, ladies of a certain age (moi) should avoid synthetic yarns, and look to linen, hemp, and lighter-weight animal fibres. And some people cannot knit with cotton without their hands cracking, chapping and bleeding (also moi).
- Clara Parkes’ Knitter’s Review website is an excellent source of information on wool, and her book, The Knitter’s Book of Yarn, is a deep-dive resource on all yarns.
- The Fleece and Fiber Sourcebook should be in your project bag, and under your arm when attending wool festivals.
- You should also have Yarn Substitution bookmarked in your browser, for when you can’t find or afford a recommended yarn, or when it’s been discontinued.