I'm too cheap to be a yarn snob, and will use acrylic, especially Lion Brand's Vanna's Choice. Wool is okay. It's a bit scratchy. I don't think I could wear a pure wool sweater. I'm just starting to use an alpaca and wool blend for my Effortless Cardigan - Berroco Ultra Alpaca Light - and it's a dream.
But I think my favorite is cotton. I love cotton yarn. It's cheap, comes in tons of colors, and can be used for everything. I normally use it for washcloths or cat toys, but it can even be used for sweaters. It's soft yet strong, and gets better with washings. I love it.