If you're newer to weaving, you may not realize that not all yarns are made equal. In fact, there is so much variety in yarns it is rather hard to qualify what are the best combination of characteristics. And it can be even harder to purchase yarn that has those characteristics. Throw in the need for a large color choice for tapestry weaving, and the options do narrow somewhat.
I live in the USA, so I mostly talk about yarns that are easily available here with one favorite exception from the UK.
So what does make a good tapestry yarn?