at one's best

at best

1. In the best possible scenario. At best, Doug will be only an hour late. They're only selling junk at the yard sale, so I think they'll make $10 at best.
2. Ideally suited for success. In this usage, a pronoun is used between "at" and "best." I'm not at my best when I'm tired.
Farlex Dictionary of Idioms.

at one's best

In one's most excellent state or condition. For example, The photographer tried hard to show the bride at her best. [1500s]
The American Heritage® Dictionary of Idioms by Christine Ammer.
See also:
  • at best/worst
  • at its best
  • best-case scenario
  • scenario
  • look (one's)/its best
  • look your/its best
  • as best (one) can
  • as best one can
  • as best you can
  • (had) best (do something)
References in classic literature
What one wants in the person one lives with is that they should keep one at one's best. I often wonder what I've done to be so happy!" she exclaimed, and a tear slid down her cheek.
The Voyage Out