What is most important part of a system?

Just wanted to add that my profuse term "night and day differences" is also most subjective. It`s dependent of different systems (rooms), personal preferences, hearing skills/abilities etc.

Isn't the premise of this whole thread faulty?  That a single component will make a system worth listening to.  For example, I often see expensive speakers paired with average components as if the speakers are going to magically make the system worthwhile.  

As much as we would love for there to be shortcuts, the reality is that the system must be properly balanced and crappy components anywhere in the chain (including the room) make it such that you might as well light on fire the money paid for good components because they will forever be handicapped by the components not similarly commensurate in quality.

The quality of recording that you are playing is the most important. A good recording will bring out the best even in a mediocre system and a bad recording will make the most expensive systems sound not listenable.

Yep, Audioman, but mediocre systems need their own recordings to sound good.
Most of the mainstream stuff is recorded crappy, so it will sound good in your car radio or iPhone ........
Agree, recording is the beginning of the signal and if it`s crap then everything below is a ................ :))

Depends on the room and quality of local electricity .