If you're in the market for king sheets, make sure they fit your mattress.  King sheets should have deep pockets to accommodate larger king-size mattresses and prevent them from slipping.  Also consider the sheet's thread count, which refers to the number of threads woven into one square inch of fabric.

There are different varieties of cotton, as well as different quality levels. Two of the highest quality cottons are Egyptian and Pima/Supima cotton.Egyptian cottonis grown in Egypt, as the name suggests. Pima cotton is grown in the United States, although you might often see it referred to as “Supima,” as well. This is a portmanteau of “superior pima” and is a registered trademark of the Supima trade association, which promotes Pima cotton. 

However, despite its pros — including the inherent water-resistance — polyester absorbs oil and grease and will leave you with the nightmare of trying to remove stains, particularly with white sheets.