Best type of metal for turntable platform?

If you're dead set on using an aluminum plate for a plinth, then the alloy makes a big difference. Alloy 2024 is recommended by some who have experimented, and the temper does not seem to matter much; all tempers seem to have about the same loss modulus, which is a lot better than alloy 6061.

Antinn gave you good advice. Constrained layer damping is an important element in most plinth designs - it’s often absent and that’s always obvious.

If you want an easy solution, I use Delignit panzerholz B25, which costs about $1000 a sheet. In my plinth, constrained layer is partly in the plies of beech wood in the panzerholz, and partly from a top layer of carbon composite.

Rigidifying your floor is even more important. My floor is concrete - it’s great - and as others have suggested, try to make your floor as much like concrete as possible.

Good luck!