Comfortable, not itchy, and air-dried sheets (that smell like they were hung out to dry on an outdoor clothesline). Either that or flannel sheets. I don't care, really, just as long as they're not itchy, lol.
Depends on the time of year and the weather but I like flannel sheets in the winter, they're warm and comfortable. Satin sheets in the summer are cooling.
Plain old cotton ones. Plaid and floral are pretty but are mainly covered all day with a bedspread. Silk if you hit wrong can put your butt on the floor.
Edit: Also I found this and I think it's pretty awesome