When the perfect thing to say is right on the tip of your tongue, you can say you're searching for the mot juste. The mot juste is the exact right word for the occasion.
Many people come up with the mot juste when it's far too late to use it, but every once in a while you'll find it just when you need it. Writers tend to care more than the average person about using the mot juste — in fact a French writer, Gustave Flaubert, coined the phrase, which means "exact word" or "right word" in French, and defined it as his own guiding principle as a writer.