heyday
Concise Oxford English Dictionary © 2008 Oxford University Press:
heyday/ˈheɪdeɪ/
▶noun (one's heyday) the period of one's greatest success, activity, or vigour.
– origin C16 (denoting good spirits or passion): from archaic heyday!, an exclam. of joy or surprise.
'heyday' also found in these Oxford entries:

