in (one’s) heyday
the most successful time or period of somebody or something
„In his heyday he was writing about 4 books a year.“
A: „Which has been more lucrative, singing or acting?“ B: „Singing, simply because in my heyday as a popstar I was getting gold and platinum albums and I was getting royalties from them selling in massive quantities.“ – Barbara Dickson
„Back in her heyday, back when she was teaching Photoshop for Photographers, she made quite a name for herself in photo restoration.“
in my heyday = za slavných časů, ve své době, v době svého největšího rozmachu