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

The list provided by ricred1 is pretty accurate although there are some others, such as the Totaldac USB cable/filter that have their followers. I own both the Curious Cable and the Totaldac USB cable/filter and I prefer the Totaldac.
BTW, regarding "heads and shoulders above the rest," the sound of these cables is system and user dependent. Also, there are many methods of getting your digital music to your DAC and there isn’t even universal agreement on the best way to accomplish that. Throw in add-ons such as the various in-line reclockers and linear power supplies and you are left with a basic crapshoot. You really need to listen for yourself.

LightSpeed 10G  usb is my favorite $999
Audioquest carbon and Supra usb cables are inexpensive yet very good
Alan

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 .