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%.

@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.

@deanshias,

As others have said , Townsend knows what he talks about. But, let me if I can expound upon what he said. If you view Figure 7 of this Fundamentals of Vibration (newport.com) , when your designing a vibration isolator, the resonant frequency needs to be less than the Hz you are trying to isolate. At resonance (and all vibration isolators have one), any vibration is amplified and this is called the transmissibility. The damping factor reduces the transmissibility peak, but reduces how quickly (the slope) the vibration is reduced after the resonant frequency. So when Townsend say a damping factor of 0.16 - that is near optimum. Townsend may very well have a curve that looks like Figure 7; and each design is limited to a range of supported weights.

Because of the low frequency (foot fall) you are trying to isolate, which has a lot energy associated with it you have an engineering problem that the number of solutions is pretty limited. You can dismiss as you wish; and spend all the $$$ you wish; but your up against some very hard truths. Good Luck.