The best material carpet is made out of is nylon. It cleans well, regains it's twist when cleaned properly, resists staining, and wears best. I can't say enough about the quality of a good nylon. Dupont makes some of the best nylons on the market today.
New to the field is Sorona, or Smart-strand, by Dupont. It is a modified poly carpet that is showing promise. It is supposed to clean well, feels cottony soft and silky smooth. It's too soon to tell, but i have a good feeling about this one.
Wool, as luxurious as it seems, is not the greatest material to make carpet out of. It's a fragile fiber, in my opinion, that is priced out of reach for most individuals. Other than to boast of it's luxury, nylon beats wool hands down.