What is most important part of a system?

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.   

Speakers

It should be obvious to anyone with ANY electronics knowledge at all, that not all parts are critical.  The critical parts are the transducers, e.g. speakers and cartridge if analog.

This why the advice to put half of the budget into speakers is both common and good.  Room treatments are also critical, but are not electronic - they will require some knowledge of acoustics.

The amp needs to be able to drive the speakers, which can be difficult if they are highly reactive or require high current delivery.

The pre-amp is a control center and at the high end can enhance certain euphonic characteristics (tubiness).

The DAC is easy to get good sound from at a few hundred dollars and there is no need to spend $35,000 on it.

But $35,000 spent on speakers can be very worthwhile -- unless you buy Magneplanars for $6,000 and like their advantages over their dis-avantages.