"At one's best" is a phrase that is often used to describe someone who is performing at their highest level. However, using the same phrase repeatedly in writing or conversation can become monotonous. Here are some synonyms you can use instead of "at one's best":
1. Peak performance
2. Optimum level
3. Top form
4. Highest level
5. Zenith of performance
6. Prime condition
7. Maximum capability
8. Ultimate potential
9. Pinnacle of excellence
10. Apex performance
By swapping "at one's best" for one of these synonyms, you'll add variety to your language and make your writing or conversation more engaging and interesting.