(Specifically man - not woman: that seems too obvious. And man only, excluding Jesus.)
Is there a traditional consensus answer to this question among those in a position to know? If so, I haven't heard it.
There would seem to be three fairly clear candidates: St John the Baptist, who Jesus described as the holiest man to have lived up to that point; St John the Apostle (and - I assume - author of the Gospel, Epistles and Revelation), and St Paul.
Of these, I do not know, but I would guess that perhaps St John the Apostle was the holiest man who ever lived, the man who achieved the highest degree of theosis: most fully, deeply and constantly communed with God.