MPT, MET caps and preferred "mix" of what's in a capacitor?

In very general terms, metallized capacitors (no matter what the dielectric material) are smaller in size, however the very thin metal coating results in some sonic changes due to the slight increase in resistance but they tend to be self-healing from voltage spikes. Metal foil capacitors (here again no matter what the dielectric material) tend to be physically larger and are not self-healing from higher voltages, but usually have preferable sonic characteristics. Non-metallized capacitors tend to be the largest and best sounding.

Here are some numbers for dissipation factor (DF), smaller is better:

Polyester (PETP) .5%
Polycarbonate .05%
Polyphenylene sulfide (PPS) .05%
Polystyrene .05-.02%
Polypropylene .025-.01%
Teflon (PTFE) .025-.01%

So, a metallized polyester capacitor might be at the bottom of the "good sound" list and a Teflon non-metallized capacitor (hardest to find) would be ideal. The choice of which capacitor depends upon many variables, such as the position in the circuit, degree of accuracy needed (EQ feedback circuit, for example) space available and so on.
A very experienced electronics designer once told me the more the negative feedback in the circuit the less the need for top performing capacitors. And, the best capacitor is no capacitor.

I hope this is helpful.

An excellent, fact-based, and level-headed synopsis, @SeleniumFalcon
Should be a sticky... actually, at many different forums.