From all the reading I've done, I agree with Shibui that everyone is going to say their mix is "the best". What matters more than components, in my mind, is proportions.
You obviously want a well-draining mix that retains water and allows for airflow through the substrate. Most popular mixes will do that equally well. Where the trick comes in, is deciding how much of each component to use. This decision is based on your climate and tree species, as well as your knowledge of the properties of the components.
Say you have a mix of akadama, pumice, and lava rock (1/3 of each). That's very well draining and popular. If you find that you're having to water 3 times a day because the soil dries out, you know that you'll need more akadama, perhaps a mix of 2/3 akadama, 1/6 pumice, 1/6 lava rock.
Same goes for other components; they can be grouped by trait and substituted in most any situation. Don't wanna use akadama? Swap it for pine bark fines. Don't wanna use "bonsai pumice"? Swap it for Perlite. Don't wanna use lava rock? Swap it for even more Perlite.
As long as you have a mix of some kind of drainage component and water retention component you'll be fine. The fact that people are growing things in straight sand shows that as long as your mix has drainage, water retention, and oxygen through it, it will work.
What I do is buy bulk components and keep them sealed in their own bins. Then I can throw together any mix I want with any proportion I want. I will never buy premix again (unless I already know it's perfect for my needs). If you blindly buy premixed soil, it's almost impossible to accurately change the proportions of components to suit your needs.
