That's a matter of preference, and probably has a lot to do with temperature. If you tend to overheat at night, you're not going to want heavy flannel at bedtime! For those with texture sensitivities, a satin or silk may be ideal - or irritating! Your most comfortable pajamas will be whatever material and design lets you rest at the right temperature, fits comfortably, and doesn't get in the way of a good night's sleep.