Wool, cotton, linen, mohair, etc. Natural fibers, ie: not man made. Beware of misleading copywriting from retailers though. Some call fabric “flannel” despite the fact it’s blended wool with polyester. Always check fabric composition. Subbing in synthetics means they’re taking a cheaper shortcut. I should add that construction and fit are also very important. Spend some time searching StyleForum and get an idea of who makes quality trousers.