Most important component??

The operative word in your question was "system". In a system, everything has to work together. So it seems that the most important component would be the ability match or put together components that are compatible. In a true "system" all the components will be important. When trying to find out which is the most important, you will have to decide what part of the system you are willing enhance and what part of it you are willing to sacrifice performance if you are willing to spend less or buy lower quality in a particular component. ThAt being said, the decision will be based on what is more important to you.

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