Yarns will drape in different ways depending on the fiber content, tightness of the gauge and treatment of the material.
Acrylic: Lumpifies, drags off of hangers and sages. I can’t tell you how many hand knitted sweaters I find in thrift stores made from Caron Simply Soft that look like a flour sack hanging off the hanger. Almost always it is done in garter stitch. It feels so nice in the ball, but when you knit it up it looses all shape and gets saggy. I will use it for baby quilts and baby sweaters but never for adult sweaters. I do use it quite a bit for softies, but I use a pretty tight gauge which it responds well too. Don’t get me wrong, I use a lot of Acrylic, I think it is awesome and Caron Simply Soft is my fav. Just know what you are getting into.
Wool- Springs back, hugs tight, shapes nice. Wool will keep its shape and it has a lot of elasticity which makes it great for garments and softies alike. Especially after blocking you have a greater likely hood of a quality finished product if you use wool or wool blends.
Cotton- Drapes into a cascading fall. I have found that cotton yarn drapes and sages a little. Not as much as acrylic, but it does more than wool so it is less forgiving. I don’t knit with much cotton at all, partially because of cost, and partially because of availability.