I think that in sports where the statistics are consistent you can compare players of different eras.
In cricket, for example, the averages of the best payers are remarkably consistent.
A good test match batsman from any era will average around the 40 mark. A great batsman will average over fifty.
A batsman who averages 99 (only one, Don Bradman), proving himself to be approximately twice as good as anybody who ever played, can without doubt be called the greatest.
Bradman, IMO, is not just the greatest batsman of all time, but the greatest sportsman.