IMHO, it would be in the following order:
- HU (quality of the original signal)
- Speakers (ability to convert the electrical signal to sound efficiently)
- Speaker wire (leaving esoterics out, this would be about not attenuating the lower-power signals in the spectrum put out by the HU / amp; matters to those who can hear / discern the presence or absence of these signals)
- Amp (ability to amplify the electrical signal without introducing extras)
- RCA (absence of injected external noise; needed only when there is an amp)
This would be the order in an original install as well as for improving an existing install. Right said, Raghu, a chain is as strong as its weakest link.