What is *truly* the very best arena footing for horses?

I think the most important factor for "good footing" is the actual location of the arena. For example, where I board now they have a lovely white sand ring. The problem is that the arena is at the lowest part of the property in already saturated ground. A stream and a marsh are about 50 feet away. More than in inch of rain and it turns into a slippery non draining mess. The old farm also had an arena at the lowest part of their property ...however, the owner spent a massive amount of time elevating it and placing drains all the way around . it was a mix of stone dust and sand on a really thick clay base. Even after a pooring rain it had great traction and no ponding. Needed frequent dragging as it would become hard as a rock in the summer. The footing that seems the most versatile, was sand & rubber bits. These two footings, together, make it dust free....as the rubber holds moisture in. I have seen euro-felt used a lot in indoor arenas. It is by far th4e most expensive footing and needs to have felt added over time as it does break down. Also, aside from a lite misting from watering systems it can not get wet. As in you can not let a horse pee on it as it really ruins it by turning it into a clumpy sticky mess.

There's no such thing as arena footing that's best for all circumstances. Your choice of arena footing depends on what activities will be done on it, the environment (indoor, outdoor, underlying soils, etc), the amount of maintenance the owner is willing to do, the budget, and whether the arena is also used for turnout, among other factors

sand clay and decomposed granite about 6 inches deep over a foot of 1/2" limestone about 1 foot thick on top of another foot of sand. Other formulas are regional favorites based on native material... you dont want dust... it has to dry and drain and it cant be too deep or too hard.

Avoid wood chips.

In the desert... we dump road apples and they dry up quickly... it works and the rocky sand makes a good sub base...

There is no such thing. It all depends upon the discipline and the climate. Indoor footing is different from outdoor in a dry climate which is different than in a wet climate. For jumping we prefer grass.

For what discipline?

Jumpers aren't going to want the same footing as reiners. Reiners aren't going to want the same footing as western pleasure riders. Western pleasure riders aren't going to want the same footing as barrel racers. Barrel racers aren't going to want the same footing as Dressage riders....