Wool will be most comfortable. It’s ’thermoregulating’ because of its hollow fiber structure, which also helps sequester moisture, and allow it evaporate, without feeling damp or clammy. Wool with 20 or 30 percent nylon or acrylic is also comfortable, and somewhat more durable. A bit of mohair adds a lot of durability and warmth, but is often scratchier than wool. Merino wool is soft and non itchy, but more subject to pilling and wear through abrasion than other wools. Sweaters knitted at a tighter gauge will be less subject to snags and runs. Wool that has shrunk cannot be restored or ‘stretched out.’ Wool does though, readily shed most dirt and stains, so don’t be deterred by grunge or odor. Don’t overlook obviously dated wool sweaters. I recently purchased a 1962 wool letter sweater from Purdue. After a wash, the sweater looked like new despite its age. Cashmere and alpaca sweaters are softer, less durable, less retentive of shape, more pilly, and a bit warmer than wool. All natural fiber sweaters should be washed immediately or sequestered in a freezer until they can be, to avoid a moth infestation of other clothing and carpets. Answer from Voc1Vic2 on reddit.com