What is most important part of a system?

I guess there are many different opinions on this topic!
I agree with the above well described weakest link theory. But to my logic and ears, speakers will impart the biggest personality influence on sound quality. Yes, lousy room acoustics, poor recording and distorted amplification will ruin sound quality and it goes without saying that these weak links must be removed in order to have even decent sound. With none of the weak links present, a change of speakers will have the biggest impact in sound. Spending extreme sums on cables, cords and other minor influences prior to getting speakers, room and source right is a fool’s folly propagated by ultra high end manufacturers who make serious profits. Speakers and room, if done right, are unfortunately the most expensive part of the equation requiring the most time, money, effort and wife acceptance. Cables and cords, to my obviously tone deaf ears, don’t make much difference relative to the cost. I have heard great speakers with average electronics and have been wowed but have been underwhelmed by great electronics paired with sub par speakers.

Speakers.....60%
room............20%
source/music/ amp.....15%
all else.........5%

Estimated margin of error 5% depending on component combinations and sound preferences.

The above is my ball park assessment of importance relative to achieving sound quality. I too cringe when I see a virtual system with a large fortune spent on components with no attention paid to room, furniture and acoustics. Take a look at one celebrity’s $500k system with marble floors, glass walls and no acoustic treatments (he must have limited upper end hearing if he thinks that echo chamber sounds right). In the end, to each his own. 

Ok if speaker/room/amp interface is a fundamental then I cast a vote for the preamp!  IME I have used lessor sources with a great preamp had very good sound. I have not however had great sources sound to my liking with a lessor preamp.  Long live the stand alone preamp!!

I think everything is important. In my system, the least expensive component is my Revel M106 speakers. Computer modeling, material advances, and inexpensive overseas manufacturing has dropped the price of quality speakers compared to high end electronics.

My DAC, Pre, amp, and Sub all cost over $2k each, and the system sounds phenomenal to me. Minor changes upstream are very obvious. If I change 1 of the 3 sets of tubes in the preamp, it's noticable. If my amp isn't warmed up, it's noticable. If Tidal turns my chromecast volume down a single notch, I can hear the compression.

I think people are putting WAY too much importance on the speakers, because you can get terrific speakers for $1500 or less, that use the same drivers as $5000 speakers.

There are so many variables that this equation is unsolvable.
Different - expectancy, taste, room etc. etc. etc.........................

But I suppose that most will agree that there are no point to buy an expensive source if you have a mediocre preamp.

My suggestion is to start building your system from speakers to up.

plutos one pair of transformers in the preamp.  As good as the preamp is, the DAC made more of a difference in every system we installed it.  In some instances it was night and day.  We were stunned at that - the light was off and then the light was on kind of difference.  The sound of the DAC was transforming no matter what speakers, preamp, amp or cables.  Same goes for our phono stage.

Happy Lisening.