Translation:Certainly it is the most fun toy.
You have it correct, although di più can also be the comparative form of 'much'. Here is a trustworthy explanation:
much .. molto .. adverb
more .. più, di più .. comparative
the most .. (il) più .. relative superlative
very much .. moltissimo .. absolute superlative
From: Italian Verbs and Essentials of Grammar, Carlo Graziano, 1987, p. 147
'Certain adverbs have irregular comparative, relative superlative, and absolute superlative forms.'
BTW - bene, male, and poco are also listed
well .. bene .. adverb
better .. meglio .. comparative
the best .. (il) meglio .. relative superlative
very well .. ottimamente .. absolute superlative
I thought the word reference for più was also was interesting to peruse, it shows a lot of other words used with it.