One trip down the yarn aisle of your local hobby store will reveal that there are a lot of yarns on the market. A lot of yarns are suitable for amigurumi. Generally you want to stay with a medium worsted weight yarn. Bulky yarn may not produce what you were hoping from your amigurumi. Light weight yarn can be used for amigurumi but the gauge may be smaller; you may have to add extra increasing rounds to get the size you want.
When looking for yarn to make amigurumis, you want to look for good, all-purpose yarn. Acrylic worsted weight makes a great choice for amigurumi yarn. There are many colors and the yarn is durable. Cotton yarn is also a good choice for amigurumi too. You will want to use a medium weight yarn for your amigurumi. This is a great weight to work with when making crocheted dolls and toys.
Before buying yarn, check your stash since you might find your yarn there. Making amigurumi toys doesn’t take a lot of yarn. If you have a some left over from another project, then try using it. Sometimes you may need two or more colors so finding yarn from your stash will save you money. If you are making a toy for a child or baby, then choose an amigurumi yarn that is soft. Using a baby yarn or cotton would both be fine choices.
Bottom line is that you can practically use any yarn for amigurumi. Medium worsted weight works best but others can certainly be used. Homespun or bulky yarn probably would be difficult to work with for small amigurumi. Acrylic or cotton yarn works well but wool and other fibers can be used with good results.