iPhone case material for best heat dissipation?
You can simply take a physics data book and lookup various materials to see how conductive they are.

Of the ones you list, metal is going to be the most efficient. For example in computers, heat sinks are often made of aluminium.

Note that there usually other properties to take into account when buying a phone case. For example it's hard to find transparent and very flexible metal (i.e. something that will allow you to bend the case slightly to allow you to put in the phone, but won't permanently deform the case).

In some cases those properties can mean that the phone won't be usable for you. For example, I know that you can buy cases with backs made of brushed steel. They are good at heat dissipation, they're flexible for you to put the phone into them easily - but the problem is they also block cell reception and WiFi signals.