I'm often asked what's the best fiber to choose for new carpet. I'm not associated with any retailer so can offer an unbiased opinion. In a nutshell, the best overall choice for most applications is nylon. It's the best wearing for a cut-pile among the synthetics, and gives a better color selection in a berber or commercial loop. It cleans very well.
Polypropelyne, also called olefin, is great in a loop for wear and stain resistance. It is widely available in commercial carpets, berbers, and outdoor carpet. Colors aren't quite as vibrant, and it doesn't do well with oil-based stains. But overall it cleans very well.
Polyester-AVOID TOTALLY. Though stain resistant, it's wearing properties are horrible. You get more weight for the money, but when you trade off the fact it will look prematurely worn, it's a bad value in the long run. And you won't find a carpet cleaner who likes cleaning it.