Where do cables upgrades have the most impact?


Assuming all existing cables in a system are atleast mid-grade (not junk), which cables/interconnects should be upgrade first and in what order? Where should one start?
128x1281extreme
I have my best interconnect between my preamp and power amp because I get the benefit in all audio sources. I have my best power cord in my power amp. 
A few good question. My two cents:

Years ago I borrowed some superior speaker cables and were blown away by the impact. They really improved the SQ.

A few months ago I purchased a new music server (Innuos) and borrowed a TELLURIUM Q SILVER USB cable with it. I later replaced this cable with a Oyaide at 1/5 at the price and the reduced SQ was unfortunately very clear. So good USB cables are giving better SQ.

A few weeks ago I purchased a net Ethernet cable (Supra Cat 8) and I must admit, I did not expect any improved SQ. Surprise, the SQ improved.

So I must admit that I have been convinced that better cables do give better SQ.

A new audiophile invited me over to evaluate his rig; I was certainly impressed by the high end names on all his pieces, but what I heard should have sounded much better since he had verifiable gear that I had auditioned in high end salons.

I was leaving town and didn't have time to discover what was wrong, but it certainly wasn't the gear he had purchased.

The only thing I noticed was that he had too many different brands of interconnects. I'm an electronics technician and I make my own interconnects using a top grade wire that has long been unavailable to the public. That happened after people like me bought the wire in bulk, and sold to those who heard about my interconnects through word of mouth. Now that I can't get the wire, that enterprise is dead.

These cable manufacturers have so many different philosophies (ground shield on one end, ground shield on both ends, don't ground shield at all, a floating ground) that when you buy from different manufactures, those different philosophies are bound to collide. Right or wrong, I'm preaching consistency; for your line level interconnects, they should all be from the same place with the same philosophy.