Why Use Aluminum for a TT Platter?

Chris/Teres,
My understanding of the technical term for cogging means either slots or
cores are required (though I could imagine where badly-designed slotless
coreless motors could have other iron present. I assume that the 'cogging'
issues you mention on slotless AND ironless motors comes from the fact
the stators are not infinitely small. Does it also come from coil material
saturation/resistance of some kind (i.e. Induction is not perfect because of
materials issues)? Is there something else I am missing?

I completely agree that in the end it comes down to execution.

Copper is the best for platter top!

Cogging is measured as torque ripple. It is the amount of variation in the developed torque as the motor rotates. A single phase AC motor by definition has 100% torque ripple since at the zero crossing of the waveform no toque is generated. DC motors have much more constant torque and typically will have torque ripple values of 10 to 15%. The motors used in DD tables have multiple overlapping phases that dramatically reduce torque ripple. In theory a three phase motor will have less than 1% torque ripple. But that is only if they are perfectly constructed and driven with perfect waveforms. But even a crappy three phase motor will have relatively low cogging compared to a single phase AC or DC motor.

Typical motors will have windings wrapped around a laminated iron core. This focuses the magnetic field and makes the motor more efficient. Core-less motors (AC or DC) have copper windings that are formed without a core. They have less cogging because there is no attraction between the magnets in and the iron cores. Cogging can also be reduced by angling the cores so that they have equal attraction between the magnets and the cores as the motor is rotated. You can tell how much the cores affect cogging by turning the motor by hand without power. Core-less motors are less powerful, more expensive and usually have less cogging.

Chris,
I was wondering what physical phenomenon generates the torque ripple in a slotless, coreless motor. Any thoughts?

Copper is the best for platter top!

There are problems with any metal for being the top of a platter. The top of the platter should be available as a means of damping the vinyl as it is tracked by the tone arm. Metal is ill-suited to this task.

Copper is a special condition that likely should be avoided. In the presence of a magnetic field (perhaps the one in the cartridge itself), the spinning copper will form a sort of primitive generator, and could easily be responsible for extra noise in the playback.