KH and GH are parameters that it can help to know, so those are good kits to have. About the only other one of value, in my opinion, is pH. I think test kits for NH4, NO3, PO4, Fe, etc. just lead you in the wrong direction, so are not worth having. For a very brief period during tank start up a NO2 kit can have some use, but not later. But, I am one who almost never tests anything, and doesn't believe in doing so, so take my suggestion for what it is worth.