It depends how much effort you want to put into washing.
Wool - less frequent washing, but hand wash with special soap and then line dry. They can take more than a day to line dry. They don't fit into the regular laundry.
PUL - machine wash, then line dry to last longer or machine dry if you're in a hurry.
Fleece - machine wash, machine dry. (line dry for less pilling, and they dry way faster than PUL.)
My favorite is fleece: Sugar Peas 2-layer fleece covers are delightfully soft. Simons Custom Cloth are really good, with your choice of colors, great embroidery options, and good prices.
The Imse Vimse Bumpy covers are great if your baby is okay with velcro.
Mother Ease airflow covers - the fit on these is not flexible. They worked great for my baby for about three weeks; otherwise the fit was terrible.