The worst thing i ever did was......這裡文法上有冇出錯?

By just inspecting the sentence, you need to use present perfect tense as you have not specified a time. However, if the completion of the sentence has a specific time, the beginning is ok, e.g. "The worst thing I did was .... on my ninth birthday', etc.

Present perfect:

'The worst thing that I have ever done was ... '

我覺得應該係
I ever did the worst thing,