If you are using staight salt then the I recommend a good quality solar salt. In my experience Morton brand is good. My definition of "good" is based primarily on being clean--very little "dirt".
The alternative is a pellet salt. There are several brands and generally they are very clean and also have an additive that is supposed to help clean the resin. The downside of pellet salt is that a binder is used to hold the pellets together and in the past this sometimes caused problems.
Small amounts of dirt in solar salt don't really cause a problem other than a gradual buildup in the bottom of the brine tank over time that will have to be cleaned out.