The phrase "in heyday" refers to a time when something or someone was at the peak of its power, popularity, or influence. There are several synonyms for this phrase that can be used in different contexts. "In its prime" is one such synonym that indicates a time of flourishing or optimum performance. "At the height of its power" suggests a position of authority or dominance. "In full swing" implies a period of energetic activity, while "at the top of its game" indicates a period of exceptional success or achievement. Using these synonyms can help convey the same message while providing different shades of meaning.