Best Platter Material

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.

The variables and combinations can get complex, as a change in weight may also affect speed stability. I like the old VPI TNT delrin/lead/cork platter, topped with a 4 lb. TTWeights copper platter, topped with a Trans-Fi Reso-mat, clamped lightly with a 5 lb. brass clamp. The Reso-mat elevates the LP on small vinyl acetate cones, reportedly of the same composition as LP vinyl. The original platter is a bit over-warm and lacking in detail. The additions add clarity.

Similar process to baking a cake.

The best platter pad I have ever seen was made by Warren Gehl, who is currently working at Audio Research. His platter pads are now pushing 20 years old but are still getting good resale prices when they show up.

The issue here is the durameter (hardness) of the LP vs that of the platter pad. Too hard = bright, too soft = no bass in a nutshell. When the two are the same hardness, the pad is able to absorb vibration from the LP. Now this is important- put an LP on and turn down the volume. The better the pad, the less you will hear coming off of the surface of the LP itself. If you think that is not talking back to the stylus you would be quite mistaken!

Once the pad has absorbed this vibration, it cannot reflect it back to the LP. On top of that, the stylus exerts considerable pressure on the LP surface! 1.5 grams may not seem like much, but given the size of the needle its like several hundred pounds per square inch! The result is that the LP depresses slightly within the immediate vicinity of the stylus. The platter pad must support this and not also depress.

Finally, its good if the pad also provides some damping to the platter, without that function interfering with its ability to damp the LP.

Most pads are too hard- any metal, acrylic, glass and the like. Of the list above, delrin is the closest. IMO its unfortunate that Warren no longer makes his pad. When he did, what we found was that as long as the 'table could manage the weight, that table would sound instantly better (better bass, smoother more detailed highs) than any table without it.

I've been thinking that the thing to do is to build a pad out of a blank LP, bonded to a damping product that has a metal substrate, perhaps stainless or aluminum. This would have the correct hardness, being the same material, and the damping control at the same time...