Mat Material...............

Since you received no responses before, I will respond, however, I will qualify this as being a secondhand response from my turntable dealer. (I am somewhat of a newbie to analog, so I will definitely defer to the more knowledgable people on this site.)
I purchased a used Basis 1400 TT, and it came with a brand new nonefelt extreme mat ($25 new, I believe), still in the package. It arrived via UPS and I brought the entire package to Brian at The Analog Room in San Jose, CA to have him set it up, as I was purchasing my cartridge from him. (His reputation proceeds him as THE expert in analog in our part of the country). When I showed him the mat, he threw it back in the box and said not to use it, that it would sound better without it.
Since I have a RB 300 tonearm, which does not have an easy VTA adjustment, I have never experimented wtih it based on his recommendation. I trust Brian, and it does sound fantastic without it. (In fact, I am going to be giving the new mat to a friend (he loved the sound of my system so much, he too bought a TT), so he can exchange it with his felt mat which came with his TT).
These are my thoughts on the matter.
Good luck in your research!

I think that what mat works best will vary from table to table and installation to installation. This has to do with the various damping characteristics of the tables suspension, the quality of the bearing / noise transmitted from the drive assembly itself, how well the table is supported, how susceptible the table is to both air and floor-borne vibrations, etc... In effect, one could change mats and use them as "band-aids" to fix other problems that the table / arm combo suffered from. The end result might be some type of colouration ( reduced dynamics, softer transients, etc... ) that is more pleasant to listen to than the original components.

As a side note, Kurt mentions a very important aspect that many people overlook. The differences in thickness from one mat to the next can play havoc with VTA, etc... Unless one resets VTA after changing a mat, they are not so much hearing what the mat has to offer, but what the differences in VTA change. As such, for consistent performance and sake of even comparisons, you would need to adjust VTA for each mat and then listen. Sean
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I think Sean is on point, there is no one answer to which mat material is best. What works for one TT may not work for another. With that being said, in the hay-day of vinyl, a lot of scientific research was done on turntables and their components. Kenwood's testing suggested sonic improvement as the platter mats progressed from rubber, to felt, glass,and ceramic mats. the use of an outer record weight and spindle mounted record weight increased the sonic performance greatly. If you accept this line of testing, the ridged mats should work the best. I use a currently use graphite mat with an inner and outsdie record weight, but intend to test a carbon fiber mat in the next week. I have also used the latest harmonix record mat with excellent results.