I'm new to canisters, what is the best media.

Sounds like you have the bio and mechanical filter aspects off to a good start. The remaining issue is chemical filtration which is usually not needed in fresh water unless you have something in your tap water that needs to come out or you have been using medications that need removal. Otherwise I would probably go with more mechanical / biological by filling the remaining space with filter floss. There also are a large number of different filter media made to address specific problems your aquarium might have.
The beauty of a large filter is that it gives you choices. The down side is that you now need to make choices and not just load the cartridges that a particular manufacturer gives you.

Ceramic rings are a good solution. Filter floss is a cheap solution .

The stuff with carbon is that you do not really need it. It will be exhausted after one or two weeks, and if you do not replace it, it will serve just as a growing place for your bacteria, like everything else. It's not bad to have it, it's just unnecessary and might slow down your water flow, because it gets dense with debris. Some filter manufacturers recommend removing the carbon after an initial phase, where the carbon captures all potentially harmful chemicals that your new equipment may have introduced.

I like ceramic rings, small bio-balls, etc for the extra compartments in my canister filters. They last forever, just a quick rinse now and again is all that is needed. And I can grab a few quickly if I need to jumpstart the filter on another tank.

I agree on the ceramic types of media, my Eheims have two kinds.
Carbon's purpose is pretty much to remove meds from the water after you've been treating fish for something. It won't remove salt, thought--water changes do that (I had someone ask me that once). Otherwise you don't need it as mentioned.

Brand doesn't matter at all for biomedia. If the media has a large surface area, including all the nooks and crannies in the material, it will work fine. You want something small enough to fit nicely into your media basket that won't crumble or dissolve when constantly exposed to water flow. You don't want it to plug up and restrict flow and right now ceramics are a popular option. As rishMak said, its really handy to be able to grab a fistfull to jump start a new tank cycle.