IMO…
1. Power cables - for power conditioner, amp, source components
2. Speaker cables
3. interconnects
4. digital cables (whatever the connection method…USB, ARS/EBU, etc.)
5. Ethernet cable to streamer
All with a caveat that you have all the critical stuff (top notch components, speakers that you really like and your room acoustics) in order. I wouldn’t worry too much about cables (isolation devices, or footers too) if any of the above critical items aren’t taken care of first. Cable swaps are easiest and most of the time the cheapest changes we tend to gravitate towards but as long as you have decent, neutral basic cables like BlueJeans or Mogami, worry about room acoustics and components first.
Just my $0.02 based on my experience and how far into this hobby I am at this point.
@deep_333 I don’t thing $100 is a point of diminishing returns for anything in this world right now…although it would really be nice if that was the case!