Not really sure there is a so called best salt mix.. I use Kent Marine. It what works for me. I know some of the LFS here will will Salt Water. I don't like the idea of using it because I want to know everything about the water and salt mix. I also want to be able to mix my own just in case I need some at 2:00 AM and all the stores are closed.
I'd invest in an RODI unit and mix you owe water. Several people will have there idea of best salt, but you have to decide what work for you.