This is the funniest joke I have ever heard.

"I have heard" is an attributive clause here, and its antecedent is "joke", that is, "I have heard" modifies "joke", which means "that is the most interesting joke I have ever heard". The leading word "that" is omitted between "joke" and "I've heard of it". Because in the attributive clause, the leading word can be omitted if it is the object of the attributive clause. Here, that is the object of heard, so it can be omitted. Due to my limited expression ability, please forgive me if I don't understand anything, and please give it to me in time so that I can correct it!