at one's best, in one's best这两者意思容易混淆。前者是“处于佳状态或境况”,后者是“穿上盛装”。例如:
1. at one's best, 处于佳状态或境况
Miss Blockey was at her best when she played the piano
布洛基小姐弹钢琴的时候发挥了她的最好水平。
I'm at my best in the early morning.
我早晨精神状态最佳。
I wasn't at my best on Monday night.
星期一晚上我没有进入最佳状态。
This was Henman at his best.
这是亨曼的最佳表现。
If Bob is at his best, we'll be able to win the most difficult game of our run-in. 只要鲍勃处于最佳状态，我们就能拿下最艰难的一场预赛。
Daniels is at his best when he's cool and direct, rather than combative and polemical.
丹尼尔斯冷静、坦率时状态最佳；而好斗、爱辩时则不然。
You're not really at your best.
你其实并没有处在你的最佳状态。
2、in one's best穿着最好的服装；盛装
Children have been all in their best during Christmas.
孩子们在圣诞节都穿上了最漂亮的衣服。
She dressed the children in their best clothes.
她给孩子们穿上了最漂亮的衣服。
They looked as if they were dressed in their Sunday best, the girls in clean white dresses, the boys in dark trousers and plain white shirts. 他们看起来像是盛装打扮了一番，女孩子们身着洁白的裙子，男孩子们一身白衣黑裤。
《柯林斯高阶英汉双解学习词典》