Best type of metal for turntable platform?

As far as metal goes the one material that has been in use for machine tools for decades is a special grade of cast iron called Meehanite. It's known for it's superior damping quality's , very important when designing a machine tool. The last thing that a machine tool needs to do is vibrate or ring, it wouldn't work very well.
I do believe it has been used before in a TT plinth, not sure who it was though.
If your going to use aluminum, maybe check out cast tooling plate. Being cast instead of wrought it should offer better damping characteristics. It's offered in many thicknesses and comes in a machined finish on both sides. It's cut to size using large CNC circular saws.

BillWojo

They are $1500 but an incredible upgrade for a TT-Silent Running Audio. VR platform. I put one under mine and the improvement in sound was the biggest change in my system short of upgrading my speakers from $10K ones to $30K ones!
SRA has two separate companies that make vibration control devices for the military and medical. Submarines and CATs!
It is clear, with over 40 patents, SRA devices allow complex machines and components to function as they were designed.

@bjw54

I have an old Sota on a walnut turntable platform that utilizes rubber balls.

I’m a bit confused. What SOTA table do you have? And why do you have an internally suspended table (with springs, if that is what it is) on a separate base? That would seem to defeat the internal SOTA suspension system.

My SOTA sits atop my rack, all by itself, and I can walk, jump, yell at, and bang on my SOTA Sapphire TT’s plinth, and it is not effected, by anything. Perhaps a bomb going on within the room might.....might. 😁

If you need vibration reduction you want an isolated wood not metal under a turntable but if you want to you should use brass as a metal nothing else.but a thick wood shelf about 2 to 4 inches thick of laminated wood like a butcher block will work best.

I would try a sandwich of Acrylic, Aluminum, and MDF to vary the density of  the materials.  A liberal application of silicone caulking between them.