1st choose your flag manufacturor. there are plenty of different flag makers in the USA but the top 3 are ANNIN AND CO, VALLEY FORGE FLAG COMPANY and CF FLAG COMPANY. These 3 flag companies are the ONLY flag companies chosen to make flags for the US government and armed services. the flags that fly over the Capitol are Annin or Valley Forge.

dont buy the cheapest flag you can buy. if you ARE strapped for cash, go to your nearest Wal-mart as these stores have a contract with Annin and co and quite regularly sell their flags i picked up a 6×4 ft nylon flag from wal-mart last year for $24.99- or have a look on ebay.

now for the type of flag.
both Annin and Valley forge have a range of heavy duty flags. these are good for high winds. valley forge have DuraTex and Annin have ToughTex. the valley forge one is made from 2 spun polyester and the annin one i think is a mix of polyester and nylon and s a 'double flag' basiclaly 2 layers of material throughout the flag. be warned though- these will just lay against the pole if its not windy enough

in a regular or low wind nylon is the best lasting flag but make sure you have at least 4 rows of stitching at the fly end which all these 3 companies do provide

for the best visual effect however a .cotton flagwill win every time in regular wind. it will flow gently and be wonderful to look at. however these flags come with a higher maintainance and they do not like the rain!! you can get colour fast cotton flags but still if it rains they fill with water and become very heavy and then have to be COMPLETELY dried out before folding otherwise it can become mouldy… yuck!
References :
flag collector and enthusiast from England