Word request: a period of time in which someone's best work was done

Solution 1:

You could consider prime:

From the Cambridge Dictionary: http://dictionary.cambridge.org/dictionary/english/prime

the period in your life when you are most active or successful

For example:

Great Expectations was the next-to-last novel of one of the greatest writers of the 19th century, Charles Dickens. It was published when Dickens was at his prime, and after all the experiences that developed his style and skills.

Mozart held a variety of positions throughout his life, but many consider his time spent in Vienna to be his prime.

You could also use the phrase 'prime period'

Solution 2:

Oxford Living Dictionaries defines heyday as:

The period of a person's or thing's greatest success, popularity, activity, or vigour.

Some examples from Google provide evidence that heyday fits well in the sentence in question:

• That's an incredible outpouring of important music, and it corresponds precisely to Mozart's heyday as a performer.

• Conductorless concerts were actually the standard practice in the Baroque period as well as in Mozart's heyday in the late 18th Century.

• The four late sonatas, completed in Vienna between 1784 and 1789, correspond to Mozart's heyday as a performer.

• Mozart's heyday was about the same time as the American Revolution.