Simply go to the store and fondle the yarn. When you find one that is so soft you can't resist, buy 250-400 yards of it (depends on the pattern you choose) and look at the needle recommendation on the lable. You can do any pattern with any yarn. Unless you're a loose knitter, I'd go up a needles size from the lable recommendation. Usually the looser you knit, the softer the garmet.
PS - I was in AC Moore the other day and ran across this new Caron yarn that had to come home with me - it was SO SOFT!!!! And you get 250 yards for $4.99!! Here's a link to see the colors: