Best type of metal for turntable platform?

+1 +1 +1 noromance

You sure got it correct.
Concrete is the answer to vibration and indeed marble and stone, and plenty of it.

OP has a suspended floor above crawl space and bare earth below. Great start.

Rolls Royce solution:  Take up the floor, dig out a bit if required, lay new DPC, in-fill with concrete screed, lower floor level to get more ceiling height if desired.  According to taste, re-lay boards except where equipment and speakers are to stand.  Or lay carpet.
OK there is a cost, but it will be worth it.

Use marble or stone blockwork for equipment supports, standing on spikes (PLEASE NOT SPRINGS - BOINNGGGGG...GGG...GGG!).  All you equipment will be rock steady, no vibration.  It is mass-loaded to Planet Earth.  Infinitely mass-loaded as I have said.  OK, not quite infinite but the mass of the earth is quite large.

This is absolutely excellent for turntables and speakers, good for CD players and amps benefit too.  There is no ground-borne vibration (except in California) and the mass damps away all air-borne waves.

I have implemented this construction in my sound engineered basement listening room.  Improvement to SQ is much bigger than many component changes.

Target wall mount TT stand fastened to a load bearing wall.  $150 and you are done.  

Best to fasten 3/4 plywood to the wall studs for the Target TT platform. Gives more positioning options and more peace-of-mind.

I would use a run-of the-mill Technics turntable as a plinth... completely immune to footfall.

What's the next problem?

BTW, there are plenty of round things that are designed to vibrate and resonate...look at bells, cymbals and drums. 

Going square is probably fine. Just decouple it and add mass (that doesn't have a very uniform grain structure). How about good old concrete?

@clearthinker

Rolls Royce solution: Take up the floor, dig out a bit if required, lay new DPC, in-fill with concrete screed, lower floor level to get more ceiling height if desired. According to taste, re-lay boards except where equipment and speakers are to stand. Or lay carpet.
OK there is a cost, but it will be worth it.

As I have stated, you can get the same results for much less, simply by reinforcing the floor joists in the area of the table. In fact, I would never do what you suggest without doing so first, because you are adding more weight to a poor situation. Again, this is not unusual in wood framed floor houses, both old and new. Renovations and remodeling is my business, and sometimes need to do this to clients homes, and not for audio reasons.

Let’s take an exaggeration; if your turntable is sitting on a trampoline, which you will also walk upon while using it, how much will you spend for a platform to remove the effect of the trampoline? That is allowing the ‘tail to wag the dog’. First, get rid of the trampoline, then see what you might need, if anything.

I did this in my house before getting the SOTA. I had a few 2x4’s left over from a recent renovation project, used about 3-4 to support the floor beneath by rack. Took maybe an hour. I could then walk, jump, etc. next to my rack which minimized floor fall issues to a great degree. I think we can make simple solutions and over-complicate them, and make them cost more than necessary.