at one's best

at one’s best (when you are at your best you are performing at your highest level of skill) — в расцвете, в отличной форме, в прекрасной форме, в лучшем виде

Example 1: At his best, he's one of the most exciting tennis players in the world. (LDOCE)
Example 2: This recording captures Grappelli at his very best.

Also:
alive and well
mint condition
be on top form
(AmE be in top form)

Example 1: "Sue gave a really good speech last night.'' "Yes, she was on top form'.
Example 2: If the Yankees are in top form there is no one that can beat them.

see also
[в прекрасной форме]
Opp.:
past one's prime (He is now past his prime.)
[over the hill]
[in bad nick]