This is where it becomes trickier. See it would be simple if all yarns were created equal and therefore, the higher the count, the denser and more durable the fabric, and therefore the higher the quality. This is true in some sense, however, over the years, retailers have cottoned on (get it) to the fact that people expect higher thread counts to be the best, and have started to cheat by using lower quality, thinner yarns and twisting them together to state a higher thread count. A true thread count cannot be achieved over 600 threads per square inch, unless extremely fine thread counts are used or twisted.
So the best thread count, we believe to be between 180 and 600. With long staple (the longer the yarn, the better the quality), 100% cotton yarns, you can achieve a fabulous quality and feel at this level. Anything below 180 will not last as well, and will be thinner. BUT the higher thread counts are not very user friendly as they are like cardboard and do not always feel nice. A friend of mine said he purchased some and he said what a waste of money (£40 each)
I would be gutted if I had paid that for some sheets and did not like them. So just beware thick is not only best.
For normal day to day sheets or quilt covers around 180 thread count is fine, try and get them with percale cotton as this is a lot softer than just normal cotton.
Hope this has helped.