Hmm given the magnitude of his success I think it's reasonable to assume Bradman is the best ever, but at the same time, people are way too quick to dismiss the idea of batsman back in the e.g. early 1900's simply being worse than modern batsmen. If you look at basically any footage of batsmen (and bowlers) from those times, I think it's plain obvious they weren't anywhere close to the level cricketers are today. And why should they be? It's just the nature of how a sport progresses - methods of success become more well known, technique becomes more refined, training becomes more intense, efficient and regulated etc. The game just becomes more professional in general.
There will obviously be some differences in the conditions and how the game was played back then, but the basic principles have stayed the same, and I'd be very confident any modern batsman could be transported back to an early era and would dominate with minimal acclimatisation needed, but I really doubt that would happen vice versa.
It doesn't mean that the early greats of the game couldn't succeed as well if brought up in the modern era from the beginning, but it does mean it's very difficult to tell just how good the players were by using a comparison with their peers analysis. It's like trying to work out how good a test player someone will be by comparing their performances with their peers in grade cricket. There might be some decent indications of whose likely to be good test players, but how they will rank relative to each other in test cricket is very hard to predict (for various reasons which I won't go into).
The saving grace for Bradman is the sheer magnitude of his success over his peers, and the fact he was literally to only one to be so successful. That, to me, suggests his success must have been a product of more than merely e.g. technical proficiencies, but also freakish natural attributes (like exceedingly good hand-eye, concentration etc.), probably transferable to any era.