There are 3 main stages of filtration, Mechanical, Biological and Chemical
Ceramic rings are great for primary mechanical filtration as they remove large particals from the water but dont become clogged easily so there is no loss of flow rate to the biological media.
Sphintered glass beads such as Eheims Efi Substrat Pro or other highly pourous media such Alfgrog or bio balls make the best biological medias as their huge surface areas allow for massive bacteria colonies to form, foam blocks dont work as well as they become clogged too easily and restrict water flow which starves bacteria of oxygen. Fine foam blocks make good secondary mechanical filtration to trap smaller particals after the biological stage.
Other media types are floss which is used as final stage mechanical media to remove tiny suspended particals and carbon, zeolite and various resins which remove toxins by chemical reaction.
I use a mixture of ceramic rings, efi substrat pro, foam blocks and KENT nitrate sponge granules in my canister filters.
