The phrase "highest development" refers to the most advanced state of growth or progression. Synonyms for this phrase may include terms such as peak, pinnacle, zenith, apex, acme, climax, or culmination. Each of these words suggests the idea of reaching the highest point or extreme degree of a particular process or achievement. Other synonyms may include terms such as maturity, excellence, refinement, or sophistication, which connote the idea of achieving a level of excellence or precision in a particular area. Ultimately, the choice of synonym will depend on the context in which the phrase is used and the specific connotations that the author wishes to convey.