Which component(s) benifit the most from isolation?

@mapman All of it to various degrees. YMMV. 

+1

Seems all with electrical circuits.

Substantial wooden platforms and racks offer a somewhat affordable isolation option, maple being a popular choice. 

I stack everything on top of the speakers and let them fight it out.

Good one!  Definitely puts you in a good place for further improvements.  :^) 

Spikes couple directly to the floor.  Energy can go to the floor as well as come back.

I use speaker isolation springs. Mainly because they were $30 a set. I have Herbies under everything else.

Anything with moving parts.  Based on your list, CD player is next

@aewarren ”Your next-door neighbor’s speakers”

Brillient answer💡

Always great to think literally outside of the box.