in (one's) heyday

In, at, or during the period of one's greatest success, power, vigor, etc. In my heyday as a stock broker, I was making millions of dollars each year, but when the economy crashed, I lost nearly everything. Everyone thought she was in her heyday when she won her gold medal at the Olympics, but her athletic career was only just beginning. I had a lot of powerful enemies when I was in my heyday as a mob boss.