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.

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]