Best type of metal for turntable platform?

If I were to machine a plinth, I’d choose Corian, rather than any metal.      Even if it meant stacking and gluing sheets; to obtain an adequate thickness/depth.      I’ve used the stuff, in a number of installs/applications, for resonance control.      If your tools/CNC router bits are sharp; it machines nicely and can be polished to a beautiful finish.      It’s resonance damping characteristics are much better than any metal.     A thread regarding plinth materials: https://www.diyaudio.com/forums/analogue-source/312473-diy-cld-plinth-design-measured-approach.html

@deanshias,

If your problem is a flexible floor, then the term footfall relates to the very low frequency (<4Hz) vibration that occurs as you walk across the floor.  In this case, mass & resonant damping with metal or anything else is not going to work unless you add so much mass (weight) that the floor is loaded sufficient not to bounce - but you may overload the floor.  

So what @millercarbon recommends with springs (or air bladders) is the best way to solve footfall unless you can stiffen the floor from underneath or avoid the floor entirely and shift to a wall mount.

YES! I have both of those issues, especially footfall. One because of the flexible floor and the other from surrounding vibrations. I have to tiptoe to avoid woofer flutter. and because of limited space I have to keep the whole setup in this area. I was ultimately never able to use a subwoofer with the turntable because the floor is so sensitive. I don't use a sub with 2 channel but sometimes I want to crank of the bass. Cannot do wall mount in this area. That was my first choice.

  If I were to machine a plinth, I’d choose Corian, rather than any metal.  

I would use the Panzerholtz, I prefer it of all.