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

Shunyata Research Alpha.. bested  more than a few contenders

With all these suggestions, good luck in your search Glory!

would love to know if anyone has heard curious evolved and fta callisto and/or shunyata venom/alpha/sigma.

i have a curious evolved but never heard anything other.  on order is an fta.  wondering if i should have gone alpha.  part of me suspects alpha will be noiseless but also tonally deficient.  not sure if that is a reasonable thought or not.

I've tried quite a few USB cables  and they are the most unpredictable of the bunch. What's good for one setup , sounds awful in the other. 
They are extremely hardware dependent. Therefore any advice with them doesn't mean much. You'll have to audition them yourself. 
One thing though, I found that my impressions were very similar to those people's  who had the same  transports as I do ( Antipodes CX-EX combo ) . The rest of the gear didn't matter much. 

I tried several and ended up with Totaldac and Curious (first version).  For USB, I thought the Totaldac cable bettered the Curious but with my gear I actually found both AES/EBU and I2S bettered the USB connection and I like I2S the best.  Antipodes DX (Ethernet) to Metrum Ambre (I2S) to Metrum Adagio.