For my money, shredded memory foam is the best. It isn't perfect, but I've had the best balance between comfort, durability, ease of cleaning, and cost.
Buckwheat was good at support, and stayed fairly cool, but the breakdown and difficulty of cleaning made it a problem. It just doesn't last as well. Only thing that was worse in that regard was feathers. Feathers get ruined faster than anything else I tried.
Pillow cases, I'm a cotton fan. High thread count cotton has the right balance of softness vs smoothness. Too soft, and you end up with bunching and wrinkles as you move. Too smooth, and you end up with your head moving too easy but your hair not moving well.
Polyfill sucks for everything except ease of cleaning.
Cotton batting is about the same as polyfill, but not as easy to clean.
Solid foam is just begging to sweat heavy, and damn near impossible to clean well. Plus the durability is iffy.
Pillow cases, actual linen isn't bad, but tends not to be as comfy as cotton. Silk is way too smooth. Satin is just uncomfortable. Synthetics tend to run hot, even though they feel nice. Knit cotton feels the best, but damn does it fall into poor condition fast.