英语翻译大概是说:这项活动是我参加过的不是最长时间的那一个,但是是让我觉得最受益匪浅的,最值得描述的那个.请尽量用好一些

This activity is not,I participated in the longest one,but it is so I think the most benefit,and the most worthy of that description.

Probably said: This activity is not, I participated in the longest one, but it is so I think the most benefit, and the most worthy of that description.

Although this activity is not for the longest one which I have taken part in , it let me learn more and was worth describing.

第一种：This activity is not, I participated in the longest one, but it is so I think the most benefit, and the most worthy of that description.
第二种：Although this activity is not for the longest one which I have taken part in , it let me learn more and was worth describing.