Sheep's wool. It provides good insulation.
Sheep wool is acquired by shearing tamed sheep - each sheep can be sheared every 10 days for 45 wool, for an average of 4.5 per day.
It can also be purchased from traders.
Sheep wool has the second worst protective stats of any textile in the game tied only other wools, and besting cloth only by virtue of proving a significantly more heat armor, though still little in compared to textiles like Devilstrand. Like other wools however, where sheep wool excels is in cold insulation, offering the 8th best cold insulation of any textile in the game. In comparison, its heat insulation is lacking at 19th of all textiles.
Thus, it is almost entirely irrelevant as protective clothing. Additionally, its poor heat insulation make it unattractive in hot biomes, except where bulk textiles of any sort are the primary requirement. Only its cold insulation provides utility in cold biomes, and in many of them other less insulative but more protective textiles will suffice. However, its ease of production and good insulation can make sheep wool parkas or skin-layer apparel an easy way to provide winter clothing to a colony, and possibly sufficient to avoid sacrificing any other aspects of their outfit. Compared to other wools, which largely fill the same niche sheep wool is likely the second worst but only marginally - with sheep only just beating out muffalo and bison in Per Day production rate. Sheep wool has the worst cold insulation and worst heat insulation of any of the wool types, though no wool should be used if heat insulation is required. The benefit of sheep wool comes from the low hunger rate of sheep, their ease of taming and the lack of need to maintain taming. This allows large flocks of sheep to be easily acquired and kept at almost any stage of the game, and those numbers are used to make up for their statistical inadequacies.
For use in furniture, it only has a somewhat improved beauty factor over other common textiles, but its rapid redundancy as clothing, the ease of acquisition, and modest but not insignificant value makes it a solid choice for improving furniture beauty and providing bulk textiles for construction. If wool production is being acquired for this purpose however, either larger flocks of sheep much be kept or the fast production rates of alpaca or megasloth wool may be superior if the option is available. Care should be taken as, like other wools in RimWorld, it is exceptionally flammable.