Which cable is most important to spend money on...

All of them! A High End System is a matter of balancing....

I've found that power cords are the most important. If you can't get good power to your components it won't matter how much you spend on interconnects and speaker cables.

Power cords. As I've found recently, every other change/upgrade you make is affected by your choice of AC components. Dedicated lines first, if possible, and then power cables, interconnects, speaker cables.
I'd also avoid committing alot of money to power conditioners early on, as they usually make hearing component differences even more difficult. A simple, but fairly transparent device like a Balanced Power Tech CPC, or Shunyata Hydra would be my choice.
JMO

1. Source to pre
2. Power Cables
3. Amp to pre
4. Speaker

You can do EXTREMELY well with inexpensive speaker wire and power cords, but not with interconnects. The important issue is to have power delivery/noise control and room acoustics worked properly prior to auditioning anything. Regarding the *order* in which cables should be upgraded I agree 100% with Thomas Heisig.

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