What type of cables make the most difference

There is only one right answer. OFC. Anything else is nonsense. Get the gauge that works for your length of run. In most cases 16 or 14 are more than enough. I use 12. But OFC is the answer.

I would say that in a good system, no cables are “transformational.”  They make incremental differences.  That being said, it also depends on what you are comparing.  For example, replacing the stock power cord on my ARC REF 6 with a Shunyata Alpha NR was a significant, i.e., more than just audible, difference.  Replacing the Alpha NR with a Sigma NR was an incremental and subtle difference.  Replacing Shunyata Alpha speaker cables with Shunyata Sigma’s was another incremental yet noticeable difference.  I noticed a slightly blacker background and a smoother presentation, particularly at the high frequencies.  I would love to do an A/B blind test though, if it could be set up, in order to see the differences (if any) by switching back and forth.  A friend of mine and I did that with a digital v. Toslink cable and we both felt that the toslink sounded better, so it would be an interesting experiment.

Based on my decidedly unscientific experience, (1) speaker cables, (2) power cables on the preamp, and (3) IC’s. Interestingly (to me), I got a decent incremental improvement by replacing the stock power umbilicals between my MSB Premier Power Base and MSB Discrete DAC with Revelation Audio umbilicals as well!

Regarding cables and many things audiophile, I'd ask and trust people who have reputations in the recording industry - studio owners. You will get quite different answers than on a site like this.

I use Mogami Gold or Canare Star Quad.

Power cords but specifically with respect to any gear that is digital like a DAC, streamer, CD transport, etc.

@audphile1 

I agree with you and Marissa.  To the point every system is different so there are no correct specific answers that fit every system.  However,  it starts with dedicated lines with proper wire like Audience Hidden Treasure.  At least 3 lines for amp, digital, analog and something like Furutech GTX NFC outlets.

Otherwise it is like fighting mosquitoes with the screen door open.

Garbage in is garbage out.