Du monde ou au monde

"Le plus grand cadeau du monde" -- The biggest present of the world (world's biggest present).
"Le plus grand cadeau au monde" -- The biggest present in the world.

I am sure that the first version is correct. About the second one I am not so sure, but that might just be me....

-- Chris (not a native speaker).

Bonjour Ann !

Actually, both du monde and au monde used with superlative forms are recognised as correct, and can be used interchangeably.
I myself feel like au monde is a bit more elegant, but it's totally subjective, and both are used equally often by French natives, and both accepted by L'Académie Française :)

I hope that's helpful!
À bientôt !