Protocol: Fabric composition and condition of summer tops, cardigans and dresses made of knit fabrics were recorded from a single wardrobe. Retail prices ranged from ~$15 to $100+, representing a reasonable range of high to low. Brands examined included Ann Taylor, Karen Kane, Desigual, Classiques Entier and INC. Longevity in wardrobe and laundry care were reported by fallible wardrobe owner.

Least durable to most durable (garment count in parentheses)

100% Viscose (4)
96% rayon/3% spandex (1)
92% rayon/8% spandex (1)
90% rayon/10% spandex (1)
60% cotton/40% modal (2)
55% cotton/45% viscose (1)
86% lycra(supplex)14% nylon (1)
52% silk/48% nylon (4)
70% silk/30% nylon (1)
73% silk/27% cotton (1)
100% cotton (4)
95%Polyester/5% Spandex (1)
92% Polyester/2% Spandex (1)
92% cotton/8% spandex (1)
95% cotton/5% spandex (1)
100% silk (4)

Further notes on results:
* 100% modal sometimes pilled hanging in the wardrobe before even being washed.
* Rayon blends lasted through a season of machine wash+dry cycles before pilling. Cotton/modal blends lasted a bit longer. Pilling could be delayed by air drying and probably could be reduced further if subject wasn't too lazy to hand wash.
* Cotton and polyester held up through frequent machine wash + dry cycles over the course of 4-5 years.
* Silk was either hand washed/air dried or never washed.
* No correlation between price and durability was observed.

In conclusion: Viscose, you are dead to me. Rayon and modal, you are on probation and only allowed to appear in inexpensive garments where you make up less than 50% of the fiber content.

Future research planned: 100% Linen tops have been purchased in the past 6 months and show good longevity potential.

Disclaimer: YMMV. Small sample size + the same fiber blend can be manufactured at different levels of quality. However there is a clear correlation within the clothes I've purchased and I hate spending the time shopping only to toss clothes within a season.

Have you noticed any factor that correlates to durability in your wardrobe?