What are the top 3 USB cables being used today in reference systems?

Review of Nordost Valhalla USB here, which is a rave ($3499)

Synergistc Research Galileo UEF USB also worth considering ($2995)
http://www.synergisticresearch.com/cables/galileo/galileodigital/digital/

I dont current use this cable in my system but have found improvements from changing out this cable in my old setup

There is no such cable.  What you really want is to ensure galvanic isolation (well known), and possibly regenerate the bit stream (not proven).

Or switch to optical ethernet.

Personally I feel if you have your spend over a $1000on a digital cable 
something is seriously wrong  in the design of your dac. 
The P.S audio direct stream dac minimizes the added benefit  of  spending to 
much on a USB cable. I have a AQ a Diamond  USB cable very good 
yes you can get a little better  but at what cost 2-3% for another $1500 ?
put a synergistic black fuse in there ,treat all connections with Stabilant -22 
and these improvements are much more cost effective .

WyWires recently came out with a Diamond series USB cable - their reference line. For years, their Platinum was considered reference level, so I'd imagine the diamond series to be a significant step up.  I've heard the Platinum in my system and was very dynamic and transparent.  An excellent performer.

https://www.amazon.com/AmazonBasics-USB-2-0-Cable-Male/dp/B00NH11KIK/ref=sr_1_9?ie=UTF8&qid=1497...


Above should do the trick


Good Listening

Peter