This would make more sense as 'son of the best father'
What precisely is "Celui" grammatically? I'm trying to find it in Wiktionary and coming up blank.
It's the genitive/dative form of "cel". See the declension tables here.
It would be nice if Duo would include this info in the word definition. It would be really useful for inflected languages. On the other hand what's that phrase?
"Everything is wonderful and nickbii isn't happy."
son would also be correct. in romanian son would be 'fiu', 'fiul' articulated
Isn't a best father a daddy in family verbiage? I used to see what duo would do and it did not like it! Maybe he should!
Daddy is an informal, diminutive term of endearment often used by a child, but would have nothing to do with "best."