Unfortunately - my impression of biggest impact is clouded because the order I experimented. I think the answer might be 'where ever you start'.
I started with speakers, ICs and power cables.
In each case they made an impact resulting in me continuing to experiment and listen.
During my experimenting with speaker cables I used mogami XLRs and the OEM power cables.
- The speaker cables had an impact on sound stage and clarity.
- IC's improved details
- Power cables improved details especially in the lower frequencies.
And for the record I would have been really happy to not have heard a difference!