Best is highly subjective no matter what the topic. What's you're criteria? Budget? Other preferences? All that plays into what one person will consider best. If leather or non-leather was universally best then we'd all be using one or the other. Clearly, that's not the case. If you want recommendations on a subjective matter then you need to be very clear on your criteria.
Failing that, there are countless threads on cases in the iPad Accessories subforum (this one is the iPhone Accessories subforum) that you can refer to.