Best type of metal for turntable platform?

@antinn , said it pretty well. You will not isolated the turntable from anything with whatever material you choose. You could actually make things worse. Sometimes a wall shelf will help, sometimes not. You have to isolate the turntable from everything about three Hz. Including the dump truck that just drove down the road. You can feel it even on a concrete  slab. The only sure fire way to do this is with a suspended turntable tune to less than 3 Hz preferably by hanging. SOTA and SME are the best at this. Sota's may look simple but they are very well made and very durable. You can also choose the wood if you like. For $10,000 you can have a perfectly suspended table with vacuum clamping, a magnetic thrust bearing, Top notch speed control and a dust cover to boot. Many of these feathers are available in less expensive table. 
There are many spring loaded devices on the market. I the dealer does not ask you for the weight of the table as it plays with recodes an clamps, forget it, move on. The springs have to be tuned to the weight of the turntable and you always go with the lightest spring that will work. 3 Hz. That is what you are looking for. Then you can kiss foot fall problems goodbye.

I would do some homework on damping properties of different materials. There is a name for it, which I forget. You can use two different materials with a damping material in between. Or look at the plinth of the VPI Prime Signature - looks like its aluminum in between two other materials. 

I to live in a very old house with bouncy floors.In my secondary system I have an old Sota on a walnut turntable platform that utilizes rubber balls. When someone walks into this room the stylus  skips. In my main system I have a VPI Classic 3 on a 4 inch thick maple butcher block platform with 4 rubber and cork anti vibration pads. I never have any skipping and can even rap on the rack without any issues. The vibration pads are the same ones that are used in the HVAC industry. These pads are very inexpensive and you will find that the same pads are being sold under some audiophile brand names priced at 10 times their actual cost!

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.