Which component(s) benifit the most from isolation?

Components that MOVE the most benefit the most:  Turntable & Cart, CD Drive, Speakers.   

With that being said, current can make tubes and transformers, perhaps even cables, move a little.  So some folks report improvements when these items are isolated.  

As some have noted, above, improvements often vary with frequency as well as isolation method, mass of gear, etc.  

Your next-door neighbor's speakers.

Of course anything that has moving parts: speakers, turntables, CD transports, R2R decks, tubes which unfortunately are subject to undesirable movements ( microphonics). 
 

Many don’t agree w/ me but I’ve found over the years w/ several different systems that if the listening room has a very solid concrete slab floor or similar, it’s best to couple your equipment to it & let the mass of the floor / earth do the job. If it’s anything other than that , decoupling can be very effective & every component will benefit from doing so. 

All of it to various degrees. YMMV. 

Assuming no tubes, the dac.