Which is the most important part of a stereo system?

Room acoustics.

For many years I believed it to be the speakers, but after experiencing a hi-end system in a room before it was (properly) treated acoustically, then after the room was properly treated, the difference was nothing short of stunning.
Again, the system did not change, only the room. I think anyone who is serious about hifi and wants the best from the space that their system resides in should correct the space before trying to correct the system to their ears liking.

For me it's my ears.  Can't do jack without them.
But so far as the hardware chain goes source components first as numerous have said

@tweak1  Terry9, and anyone else who thinks cables are least important:
you couldn’t be more wrong"

So you think that cables are more important than the speakers? Amplifiers? Preamp? Phono? Turntable? Tonearm? Or cartridge?

And what is your evidence? I mean a real test, not a gut feeling.

I did the test. I built a special cabinet to be a Faraday cage, then used theoretically optimal 99.99% pure silver conductors inside teflon tubular insulators which barely contacted the conductors, which were separated by 50mm for near-zero capacitance. ETI connectors. Compared to premium microphone cable with ETI connectors. Difference negligible.

The specific model of epitaxial output transistors, from the same manufacturer, same technology, and same family, made far more difference. As did the resistance of the amp's circuit breaker. Lots of things make a difference, and even cabling does to some extent. But nothing compared to an entire component. And next to nothing in terms of benefit related to cost.

terry9

I did the test. I built a special cabinet to be a Faraday cage, then used theoretically optimal 99.99% pure silver conductors inside teflon tubular insulators which barely contacted the conductors, which were separated by 50mm for near-zero capacitance. ETI connectors. Compared to premium microphone cable with ETI connectors. Difference negligible.

>>>>>Strictly speaking, that is not a scientifically valid test. That’s the problem generally with cable tests and cable comparisons. There are variables that are not accounted for in the test. Examples:cables not sufficient broken in; cables not cryod; cables’’ directionality not accounted for; the possibility of errors in the test system. Therefore I am inclined to throw your test out.