Best speaker cable for Focal Sopra 3

It really depends on whether you are working to correct something about the sound that you do not like. If you love the sound then I strongly recommend Kubala Sosna cables. I believe the represent a baseline from which you can judge equipment by. Check em out. 

One user i came across used Telluriumq Q blue diamond between his gryphon diablo 300 with good result.  The blue diamond supposedly has a laid back high and quite forgiving on slightly bright system.  I am using voodoo definition with my diablo 300 and sopra 2 seems pretty good as well.

YMMV, but I am using Morrow SP6 speaker cables with my Focal Diablo Utopia III's and I like the way the speakers sound with them over the MIT S.3's that I was using. . .

I used Inakustik with mine to great success. They are tremendously well built cables. German made and it is easy to tell. High tech high quality materials. 

Good question for Jacques Mahul - he would probably point to a simple 12 awg cable.