Grainiest developer I ever tried was Gainer's Metolal:
But one needs the chems and quite a bit of film to be developed to make it worthwhile.
Both Metolal and PaRodinal have excess sodium hydroxide that swells the emulsion but my impression was that Metolal gave larger grain.
If you just add a lot of sodium hydroxide to Rodinal it's not good, the result was mottling in my test.