At the top of the tree
Definition 1 of 2
Pro Tip 1/3Career Success
Used to describe someone who has achieved the highest possible position in their profession. 
After years of hard work, she is now at the top of the tree at her law firm.
Pro Tip 2/3Reaching the Peak
Implies that the person has reached the pinnacle of their career, often after a long ascent. 
He had finally climbed to the top of the tree, becoming the CEO of the company.
Pro Tip 3/3Elite Status
Suggests membership in a select group of people at the highest level of society or their field. 
As a Nobel laureate, she's at the top of the tree in the scientific community.