I presume every viable salt has been tried as some time in the past.
There are double salts that may be viable as well, though, so it could get complicated to test everything.
I think at least some of the final appearence is related to working methods and developer chosen, along with the paper, so there are many variables to lock down to do a comparison test.
I compared sodium, lithium and ammonium at one time and noted the differences in the results. Each has it's merits for specific things, but I generally liked the look of the ammonium version for general work.