Any liner is a good idea as it allows you to wash it and not the bag. Fleece are great if you are staying in one place, but bulky if moving about, and can be used alone in the summer or on a mates floor. If you are moving about or space is a premium silk is the business. Packs small, warm, even when wet, dries easily etc, but expensive. Can also be used in the summer and great if you get shot with an arrow whilst sleeping

. Cotton is OK but is not as warm as silk, and more bulky. very cold when wet and takes an age to dry.
You pays your money etc.
I have a fleece liner, if I could afford it I would have a good silk one.
R