so many factors to consider. true fielding is at a much higher standard overall nowadays, but to counter that bradman was terrific at finding the gaps so if he was to play today and he still had that ability then he'd still pierce the field i'm sure enough times to match the other great batsmen in history. let's not forget his average is far and away the best ever by a long way, even if you cut it completely in half he would still be there abouts when talking about great players. he had the ability to hit big scores consistently to and he also had a great repertoire of strokes, and although it hasn't been officially recorded i've read that from records that people have been able to gather and calculate that his strike rate was actually in the 70s which is very high for test cricket. his lowest ever test series average was something like 56 from the bodyline series.
so i think if you factor everything in, even if you cut him down 1/3 or even more he's still matching it with the best ever so i'd say he is the best batsman ever, he would do it in this era to.
but at the end of the day you can't really compare across era's.
All Time Test XI: 1 Hobbs 2 B.Richards 3 Bradman 4 Pollock 5 V.Richards 6 Sobers 7 Gilchrist 8 Procter 9 Miller 10 Marshall 11 Warne