SB 4.23.30

ittham-bhūtānubhāvo ’sau
 pṛthuḥ sa bhagavattamaḥ
kīrtitaṁ tasya caritam
 uddāma-caritasya te
Synonyms: 
ittham-bhūta — thus; anubhāvaḥ — very great, powerful; asau — that; pṛthuḥ — King Pṛthu; saḥ — he; bhagavat-tamaḥ — the best among the lords; kīrtitam — described; tasya — his; caritam — character; uddāma — very great; caritasya — one who possesses such qualities; te — to you.
Translation: 
Maitreya continued: The greatest of all devotees, Mahārāja Pṛthu, was very powerful, and his character was liberal, magnificent and magnanimous. Thus I have described him to you as far as possible.
Purport: 

In this verse the word bhagavattamaḥ is very significant, for the word bhagavat is used especially to refer to the Supreme Personality of Godhead, as the word bhagavān, “the Supreme Personality of Godhead,” is derived from the word bhagavat. Sometimes, however, we see that the word bhagavān is used for great personalities like Lord Brahmā, Lord Śiva and Nārada Muni. This is the case with Pṛthu Mahārāja, who is described here as the best of the bhagavāns, or the best of the lords. A person can be so addressed only if he is a great personality who exhibits extraordinary and uncommon features or who attains the greatest goal after his disappearance or who knows the difference between knowledge and ignorance. In other words, the word bhagavān should not be used for ordinary persons.