Best material for an isolation platform?

From My experimenting several yrs ago, the harder the surface, the brighter the sound. Wood is popular in instruments because it is more neutral, though often warm sound. Then there are tone woods of which hard Maple is a favorite. I suspect MDF became popular with people such a speaker Mfg's because solid wood has its own set of problems and is expensive too. I have worked with wood for 50+ yrs and love it, yet respect its characteristics
Oak should be no problem, especially at 1 3/4" as long as it is dry. Its the thicker slabs which are questionable, mostly due to drying issues. So, enjoy the oak

Best material for an isolation platform?

Half inflated scooter innertube under a stone slab/tile. 
https://www.walmart.com/search/?query=scooter%20inner%20tube

Next best is Sobothane under a stone slab/tile

Forget mega $$$$$ audiophoole items, do your own

Cheers George

@tubeglow helped me set up a new system last year, including a beautiful Starr sound single shelf rack and audio points. With direction, I was easily able to place my amp And CD player. Thoughtfully designed, this single shelf rack has significantly decreased noise and interference in my system. It’s not only audible--The difference was jaw-dropping, even with my elementary gear! Bob is an absolute sweetheart with a heart of gold. ❤️ Wish he could of known you. 🌹 Check it out on YouTube: https://youtu.be/1y1Vi8wH07I

I'm a big fan of BDR. Used extensively in my system.

Hi Krissy, good to see you.