There are several thing besides thread count to consider.
Higher then 350 good. The higher the count usually the better, if it is 100% cotton.
Check what type of cotton- Egyptian or Sea Island are 2 of the best in the world- that means that the cotton fiber from the boll is longer and that makes a thiner finer softer thread, usually.
Check the weave- Satin means that there is more thread on the face of the fabric- that makes them a little shiny, and usually soft. Because there is more thread on top of the weave they can wear out a little faster then a plain weave. But it won't make a great deal of diff on high count fiber.
A Jaquar(sp error) weave looks like brocade. This is usually a mix of satin and plain weave to make a pattern. Wears well, usually very nice sheets. Lots of $
Flannel sheets that are double sided have nap(fuzzy stuff) on the top and bottom of the weave and get very, very soft and warm.Get cotton, as poly and cotton pills a lot.
Dolby and waffel weaves can be a little rougher then others. This is usually due to the way fiber shrinks, if there is un even shrinkage it can cause a waffel effect and some people don't like this.
Get cotton, it usually won't pill, will last longer, wear well, and insulates well. When it is hot it wicks moisture away, and when it is cool it insulates you well. It also releases stains well.
Silk is great, but doesn't last as long as cotton. Real silk does NOT need to be dry cleaned. Dry cleaners have been around +/- a 100 years? Silk has been around at least 5000 years.....
it will $cost$ you lots.... but WOW. Beware of polyester possing as silk. And yes, you can slip around a lot on silk satin....Silk will stain easier then cotton.
Linen is a wonderfull fiber for sheets. They will last more years then you want. Hard to find. Most people insist they be ironed. They can be a little rough if not ironed. They get softer with a lot of ironing. And remember how cool a real linen suit/dress/ pants et are. Linen is usually dry cleaned, but I have some I wash and do not iron, I like the feel after a LOT of washing... releases stains well.
You can also find sheet that are a mix of fiber. I like the cotton/silk. Cool, sexy, washable. And you don't slip out of bed. I have seen some of cotton/linen and when I can find some I wil get them....
Most characher sheets are 60% poly 40% cotton. Remember that polyester is made of a plastic like compound, it melts into hard pills when burned, and smells like burned , well plastic. It will bet ugly when washed after a few months.
So sorry for the length, I have a lot of fabric, love it all.
Hope this helped a little.