Wow- Important question with SO many ways to view and answer it!
Importance by list rank, #1 being highest:
1. Speakers & speaker placement**
2. Amp & Preamp (one is just as important as the other)
3. Source
4. Wiring
** I am a near-field listener, so the room has little relevance other than insuring there is no echo and that there is nothing in front of or near the speakers that could interfere with them.
Speakers will always be the most important to me. What good is everything else if you don't have a way to enunciate properly?
As for everything else, "You don't put a $50.00 saddle on a $10.00 horse". The quality of everything else should follow suit with the speakers. Quality does not mean expensive, just that all other components should have decent specifications and be able to do what is expected of a system headed off by the quality of the speakers you're using.