Most important component??

Spousal approval.

Reality is, every component can significantly improve or limit one's system.

I agree with my Cache Valley neighbor, Duke about room/speaker interface and amp/speaker synergy being crucial. Preamp/amp, particularly proper impedance and sensitivity matching, is very important.

Been there and done all that at a costly learning curve.

I'm now upgrading my digital source and hear significant differences between various design approaches and price points in my own system (not at dealer showrooms). These differences are no less profound than imposed by any of the components downstream.

Just the way it is...

Bob_reynolds, agree 100%
I have found that in order of imnportance:

1)Speakers (assumed appropriately sized to intended room)
2)Speaker placement w.r.t. room and listener
3)Amp
4)Preamp
5)Digital source
6)Speaker cables
7)CDP/Pre Interconnect
8)Pre/Amp Interconnect

I love it--THE ROOM. Now what is really interesting is that about 4 years ago we asked the same question on a different site. The result was speakers, source, speakers, source, amp.... You get the point. 1 in 100 said the room. Now, most people say the room. 50% of the sound we hear in a typical room is indirect--reflections off the room (or modal energy). The room is huge--and as has already been said, the room / speaker interaction is essentially a circuit. Having the right speaker in an appropriate room is key to a great end result. It's great that people are realizing how important a role the room is--it really is a component. The first one we start with and it used to be the last one people would think about--not anymore.