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. © 2015 Farlex, Inc, all rights reserved.

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. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
See also:
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.