The "Bradman is best" school of thought essentially takes his superiority over his peers as its yardstick and no one can argue with the fact that his overall career average is so much higher than all comers
It isn't that clearcut though - when WG scored his 50th century he had scored 31.44% of all the first class centuries scored by that time - the next highest was the long since consigned to obscurity Harry Jupp with 10 - I would suggest that is an even greater degree of dominance than Bradman demonstrated.
Would Grace have scored as heavily as Bradman on 1930's shirtfronts? Who knows - perhaps not
Would Bradman have eclipsed Grace in Victorian times on those "sporting" wickets? Who knows - perhaps not
One of them is the greatest batsman who ever lived - but which one? - personally I think its Grace but doubtless few will agree with me