The word "stackest" is the superlative form of the verb "stack," which means to arrange items one on top of the other. There are various synonyms for "stackest," such as "piled," "heaped," "stacked up," "built up," "collected," "arranged," and "assembled." Depending on the context, one of these synonyms may be more appropriate than the other. For instance, "piled" may be used for a heap of books, while "built up" may be used for a tall stack of boxes. The word "stackest" can also be used metaphorically to refer to a large amount, in which case synonyms like "abundance," "profusion," and "excess" can be used.