What speaker cable made a major difference in the sound quality of your speakers??

Edward- you are completely correct. Having components that play well together, well made speakers that the components can handle and acoustic treatment of the listening room all impact the sound of a system more than exotic cables will, as long as the basic cables are of sufficient gauge and quality. But once all the pieces are in place, cables can help bring out even better sound.

Nelson Pass did some interesting research on cables and published an informative article. I made my own with 6 strands of OFC 16G, silver solder, and gold plated terminals. I like them. Better that the 12g I used and you can tell. I can't see spending $2000 on cables. That's nuts. 

Don't forget or underestimate the connectors. In my present system, and all through it's iterations, Zu Event Mk I gave me the best sound. Getting them at a bit more than half price went a long ways as well. But the best sound I've gotten from them was when I had Zu swap out the spades for the bananas on the amp end. 

The connection may have not been the best because of the five way binding posts but it was the change of metal that elevated the sound.  Simply going from copper to rhodium over a berylium core gave me a couple orders of magnitude more detail, separation, clarity, and focus.

Even the bass was better resolved and more defined as well as a better front to back soundstage due to the more pinpoint imaging. It makes me wonder if I had some of my first cables terminated that way, would I have every looked for more.

All the best,
Nonoise

Thanks for the responses.  I'll look for that article by Nelson Pass and give it a read.  I'm moving now and will be reconnecting my system.  As good a time as any to try some different cables after I've treated the room. That can get costly quickly doing trial and error so perhaps I'll begin by tinkering with different metals and go the diy route myself.

While the price point I am going to mention has nothing to do with the question the OP asked, perhaps it can shed some light on cable differences.

As a new Audioquest dealer, I had the opportunity to audition their two top of the line speaker cables on my system.
The wel signature and the wildwood.
The only difference in the cable was some of the Silver strands of the top of the line Wel were replaced with copper in Wildwood. There was still a significant amount of silver in the Wildwood and the winding topology and dialectic active bias was the same.
The audition started with the wildwood and it was fantastic, providing details and timing cues I had never heard previously.

Upon going to the Wel, there was a significant upgrade beyond what I heard.
So my point is that even at the very high of cables, where few compromises have to be made, there are benefits to be had that are not subtle.

I did the test for a golden eared installer who found the same, and I read another opinion that stated the same finding among the 2 cables.