What's the best (or is there a "best") DAC input to use?

Very enlightening responses by all. Sincerely appreciated. Besides the fact that the DAC itself often wouldn't have every conceivable input, the transport also wouldn't have every conceivable output. Amazing how this hobby's possibilities and the quest for audio nirvana is seemingly never ending, because given the responses thus far, finding the right combination of DAC, transport and cabling between the two can be rather challenging. 

I2S IN THEORY CAN BE BETTER THEN ANY TYPE OF USB
For on the 5v wire potential electrical noise ,depending how well it is isolated.  If your dac does not depend on this method  then 
It can be as good  . Ps Sudio has tun this debate for awhile.
EVEN their very gifted Digital engineer Mr.Smith agrees 
Both are close .BNC can be excellent also everything comes down
To how well the Digital products deal with every facet of the  signal. Both Digitally,and Electrically.

@audioman58  perhaps then I should ask if in this case *theory* translates into any real perceivable audible advantage. And @mofimadness , I wouldn't have the luxury of trying 100 DACs. I'm trying to get a potential purchase right the first time. I suppose what I'm wrestling with here is if I'm to buy a reasonable good DAC (like the Shiit Yggy) and then use a reasonable good transport (like my Oppo UDP205 or maybe even a Blue Sound Node 2), do I now need to be *overly* concerned with how I connect the two? 

And when I *look* (not buying one anytime soon) at high-end DACs like a Lampizator Big 7, even they are not offering an option to add I2S or firewire as an input option. So what commercially available DACs have that input?

I also have tried them all with my Master 7 dac and I2S is the best. The rest are close but I would avoid Optical or Toslink . By the way I use  a standard usb cable from laptop to a Singhzer USB converter. Then go from it with an Eithernet cable to my dac. If the dac your going to buy has BNC coax I found that is also very good. Also follow for starters the suggestions of the dac manufacturer
Alan

I2s, would never go back to usb