Which is the most important part of a stereo system?

Deschutes, why not DIY your electronics? I would say that the main thing is to stay away from teflon insulation, because when teflon heats up past about 450F, it begins to decompose into some really evil gasses. And unless you have a fume hood or equivalent protection, well ...

What @almarg said. I suggest not spending disproportionately on anything that (1) wears out, or (2) has low benefit per unit cost, like cables.

@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.