Welcome to the forum, Syazain.
We usually say '...the funniest joke I have heard...' or '...the funniest cartoon I have seen...'
It's the same as 'for a long time'.
'For a long time' is an indeterminate length of time since you last heard/saw such a funny joke/cartoon.
Please note that a better title would have been In a long time.
Extract from the Posting Guidelines:
'Thread titles should include all or part of the word/phrase being discussed.'
Student or Learner