He is at his best when he speaks off the cuff.

他即兴讲话讲得很出色。

　　off the cuff: without preparing ahead of time what one will say; extemporaneous. 无准备地，即兴地，即席的，不用讲稿的
　　再如:
　　Considering that the speech was off the cuff，it was rather good.
　　由于这发言是毫无准备的，所以它还算是不错的。