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

I agree with the common thread that it's a question with no definitive answer.
Too many variables: equipment, interconnects, acoustics, your personal sonic preferences and even whether or not you have an "ear" to hear the difference.
Like most longer responses, it's been a journey through equipment, interconnects, speakers and then speaker cables. For me personally I like the sonic signature of Yamaha combined with MIT from receiver and components to speaker. I use MIT MH 750 Shotgun's to a pr of Vandy 3A Sig's. for the most natural true to life sonic experience I've found.
If I could possibly do a flip a switch A to B from where I started to where I am now there would be an OMG never heard that moment.
Read all the reviews you can and pick something from main stream brands where people are describing what you want to hear in bass, details, sound stage, listenability, image, accuracy, etc.
Good luck.

Actinote Aria. I've been through many cables for my Duevel Bella Luna Diamantes and these are my "done" cables.

best ever was Atlas Asimi speaker cable and interconect also asimi . but not in my acceptable price range, i never heard anything like that, in bass and tweeter regions. but price of calble is insane :(
and interconect i managed to get of Asimi line;)
next best is silver HGA x32.
tryied MIT and other cables on my tube rig and tannoy kensington GR speakers, and compared to any silver cable, copper wires always were dry ,dead sounding. so rule how much pay ,that you will get i believe is corect

Tried many cables over the years ... some cheap ... some expensive
Finally found the right fit

Clear Day  Double Shotgun

Been with them for a few years and will not entertain any other

My speaker cables cost more than your price range and were purchased new but honestly they make absolutely no difference at all to the sound. Extremely solid and the highest quality robust connection is all I have observed from using industrial/military/aircraft grade style connections.