Perceived softness has a lot to do with the diameter of the fiber. The finer the fiber, the softer it is. Durability has to do with the length of the fiber, or the staple. The longer the fiber, the more durable it is. There is a lot of variation within each type of fiber, so the above scale is not perfect or exhaustive. For example, very fine wool can often be softer than lower-grade cashmere. This is why the fiber content can be so deceiving. A manufacturer can label their sweaters 100% cashmere, but if they are using the lowest grade they can find, those sweaters are not worth their weight in fine wool. Many manufacturers even try to disguise bad quality fiber with dye. Be wary when merchandise is only sold in darker colors.
The real point of my writing this little review of luxury fibers is to bring attention to Alpaca. Alpaca fiber is shorn yearly or every two years from the animal with the same name. Alpacas are in the camel family and can primarily be found in Peru, although the population of Alpacas in the US is steadily growing. Alpaca fiber, especially the fiber from the first shear of a baby “cria” is incredibly soft and smooth. Good quality alpaca far surpasses a lot of cashmere in its luxuriousness. It also might arguably be a more sustainable resource. Alpacas require very little space to thrive, and they are not as destructive to their environment as are goats (cashmere comes from the Kashmir goat). There are reports that Kashmir goats are devastating the environment in China because of low regulation and high demand for cashmere throughout the world.
The irony of the demand for cashmere is that alpaca is often a better option where luxury fiber is needed. The only drawback is that alpaca is heavier than cashmere, but because of its warmth, it can be spun thin to reduce the weight. It is also more durable than cashmere because it has a longer staple. Baby alpaca is especially wonderful stuff, since it is often much softer than cashmere, and can be lighter than regular alpaca. Vicuña is even better, but hard to come by since it is highly regulated.
The main point to remember is that there are so many different levels of quality within each fiber group. Very fine Llama can rival some alpaca. Very rough wool can be as unpleasant against the skin as poorly processed or bad quality hemp. And in that same vein, good quality hemp can be as soft as some cotton. It is all a continuum with no hard and fast rules, other than “buyer-beware.” If you are shopping online, enlarge the picture as much as you can to get a sense of the loftiness of the fabric. You can sometimes see the softness! Read the seller's descriptions carefully, and look for words that might indicate poor quality. For example, "Peruvian wool" is sometimes used to refer to low-grade, inexpensive wool. Price might tell you something as well. A very low price for a luxury fiber might mean that fiber is of low quality. This is not always the case, however, as illustrated in the competitive world of craft where prices and profit margins can be incredibly low considering what went into the merchandise (but that's a different article entirely). To get the best bang for your buck, educate yourself on luxury fibers and trust all of your senses to guide you to the highest quality.