This sentence is not a negation. Jamais here means "ever" instead of the usual "never" when used in conjunction with ne..jamais.
If you are questioning why the subjunctive of 'avoir' - ayons is used in the sentence , it is a rule that after superlatives and after le premier, le dernier, le seul, le meilleur (here 'the funniest' ), the subjunctive will follow because there's a doubt that in fact it is the truth...
C'est le plus grand marteau que j'aie jamais acheté = That's the biggest hammer I have ever bought
C'est le plus beau magasin que j'aie jamais vu = That's the most beautiful store I have ever seen
Arthur est le seul qui ait proposé de m'aider = Arthur is the only one who offered to help me
C'est le meilleur repas que nous ayons jamais manger = That's the best meal we have ever eaten
N.B. If there's no doubt whatsoever, sometimes the indicative will be used .
Hope this helps!
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