"Diddliest" is a colloquial term that can be used to describe something that is very small or insignificant. When searching for synonyms to replace this word, there are various options available that can be used interchangeably. Words like "tiniest", "smallest", "wee", "minuscule", "microscopic" and "petite" would all be viable alternatives. Depending on the context in which it is being used, other synonyms like "insignificant", "trivial" or "negligible" might also work well. Ultimately, the choice of synonym will depend on the tone and setting of the text being written, but there are plenty of options available to replace the word "diddliest".