Best Platter Material

Boston Audio graphite mat works really well with Nottingham Spacedeck. Its platter is made of..I have no idea, but it's quite heavy and not glass or acrylic or delrin. Some sort of alloy.

I think the alumimum/lead sandwich of the VPI TNT made an excellent platter, but the platter interface with the record is a different story. for that I use and recommend Delrin, which is also my second place choice for the entire platter.
You are asking for two different things. One, which material makes the best platter and two, which material interfaces best with the record. The answer to those questions are likely two different materials, as I stated above.

Living voice mystic mat is very good.

Does all the things that my copper top on the TW Raven AC-3 does except removes the slight edge that can afflict the copper top.

The Kenwood L07D comes with a 5-lb stainless steel mat, a la the one Albert describes. I am a big fan of my L07D with this mat. I also like the SAEC SS300; like the M-S mats, it's no longer made but usually can be found on Audiogon or eBay for sale. Boston Audio Mat1 and Mat2 would be in my top 3. So, interestingly to me, Albert and I are in nearly complete agreement. (If I ever audition an M-S mat, I am sure I would like that too.)

As far as the stock rubber mats typically supplied even with the best vintage turntables, I feel about them the way Joan Crawford felt about wire clothes hangers (if you remember the scene from "Mommy Dearest").

But I thought the OP wanted to talk about platters, not platter mats. To that I would say that if you use one of the above mats, the composition of the platter per se becomes much less relevant.