I've not investigated the heat transfer properties of various materials used so I really cannot give you a definitive answer. I will however give you my opinion on the matter.
I've edited down your post to what I feel is the most important part of your question. What exactly are they made of? the answer, your guess is probably as good as any.
This being the case it is easy to see why Mica insulators get my vote. they are easily distinguished from other types, they are very thin, and almost transparent, so it is easy to observe any imperfections which may lead to break down. and Being that they are very thin they reduce the insulation effect to a minimum and allow decent heat transfer.
Now as for the other types, and are they any good... don't know, but why risk it for such inexpensive parts? In the past some of the materials used have been horrible, hence the "90 day wonder" tag, I'm sure thermal transfer pads have progressed over the years, and some of them are probably pretty good. I believe the manufacturers use alternatives to the Mica insulators to save a few pennies on each radio.
Even some of the mica insulators are crap, the 29 I posted about in another thread appeared to have some kind of shredded and pressed Mica insulators, no doubt to save those few pennies, but they were shit.
Now I really don't know what all those other insulators are made from, and I really don't care, I would replace any of that crap with Mica at the earliest opportunity. I do know they have some type of silicon thermal transfer pads which are very good, but I can't tell it from the crappy rubber, or cheap ass plastic ones from some high quality thermal transfer material.
