Best type of metal for turntable platform?

If you view this document -  Documentation of damping capacity of metallic, ceramic and metal-matrix composite materials (utexas.edu), it gives an excellent summary of the damping characteristics for metals.  Table III is the one of most interest, and the larger n^ (column to left of Remarks) the greater the damping. 

Aluminum by itself is not that great, so unless you revert to just mass (weight) as @slaw says two layer of AL w/damping can improve the damping considerably.  Also, viscoelastic damping can be any thin film plastic that can stretch at room temp.  Ideally, the two plates are different different thickness (and different stiffness) so that the thinner plate is the constraining layer that along with the viscoelastic damping are what damps the thicker plate.   The viscoelastic damping layer can be as thin as 0.015" and be very effective.  If you then add another layer - its damps even more, but the first layer get you about 85%.

Modellers Clay such as the Brand Newplast  has exceptional damping properties and will be quite valuable as a material when used with 
materials that have structural and mechanical properties.

   

Its not the metal. There are no metals that are good for vibration control. The reason you see all these metal racks is the same reason for all the MDF speakers- these materials are inexpensive and easily machined.

The place where you get the most benefit isn’t the material, it is the design. Straight lines and 90 degree corners, these are used for the same reason as metal and MDF: ease of manufacture. For a one-off project you will get much better results using curves. No straight lines. No parallel shapes. Look at a tuning fork- straight parallel aluminum. Rings and rings all day long. That what you want? No. So no straight lines.

Your best bet will be to incorporate springs into the design. Springs are by far superior to anything you can do with cones, spikes, etc. The trick with springs is to determine the right amount of spring stiffness for the component load. I would get Nobsound springs, as they are small and extremely cost effective, making it easy to add or subtract springs according to whatever load you wind up with.

They are only about 1" high, about 1/2" under load. Then design your shelf. It can be MDF or a sand bed. Using CNC you can machine a sand bed only about 1" deep (thick) to fit within the rack frame so that it looks almost like one piece. The frame itself should then be gently curved across the front, with a tighter radius at the corners. The front, back and side rails should be flat on top but curve like a bridge arc on the bottom.

If you will be using Townshend Pods under your component then you can eliminate the Nobsound springs, incorporate the sand bed into the rack, and make a simple 1/4" plate of aluminum to cover the sand bed. Powder coated this will look rad and work like you won’t believe.

By the way, I have no idea why you're talking such tiny dimensions. Unless your turntable will be on the floor. Which works great, but incredibly inconvenient. If you want a turntable rack that is at normal comfortable height, relatively easy to build and incredibly cost effective, let me know I will tell you how to build this one. https://systems.audiogon.com/systems/8367

At the moment this platform is for a Project RPM3 with Hana SL. Im planning on upgrading both in the future. I have a wooden floor with a lot of play and too many vibrations. Im currently using a maple wood platform at 16 x 13 x 3 on top of SVS subwoofer feet and was going to put the 2nd metal platform on the bottom (maybe on top) of it. Thought if they were same dimensions and flush it would look and sound better. I was planning in putting something in between also. I have a friend with a CNC machine,  so i can pretty much make any design.

These are great pointers!