Which component(s) benifit the most from isolation?

Anything with tubes would be the 1st. 

Isolation does not have to be expensive.  I remember one guy used canned tangerine containers under his turntable. Cutting boards from Ikea?

As far as audiophile methods, buy a set and move them around.

I think one's goal should figuring out how to decouple in a smart way.

Pretty much everything in two systems use HRS isolation products , some come from factory w HRS built in ( Aesthetix ) but as a slaw mentioned above there are less costly methods notably springs and a seismic sink ( sand )’or even a combination including added mass…