Well, I don’t think the 3rd edition is the best, but I can give you my opinion on where I think 3rd edition bests the previous editions. Since the question you pose wears an air of positivisms towards the 3rd edition, I will not mention where I think the 3rd edition fails, neither where I think it fails to evoke the atmosphere of the Old World.
What I find greatest of the 3rd edition are the submechanics for Critical Wounds, Corruption, Disease and Insanity (mechanics for the recovery times would need a bit more of thinking but this is minor). A RPG for the Old World needs these kind of rules. While previous editions have rules for this, their approach is by far worse than that given in the 3rd edition, which I confess I find brilliant.
I also find the career system better in this edition, more flexible.
I think that the spell casting mechanics for wizards is the best of the three editions (not the spells though).
The submechanics for fatigue and stress are also a great addition for combats as compared to previous editions. Sadly, they do not work so great for outside combat situations (minor issue though).
Since it will be a bit silly to highlight the 3rd edition without mentioning the main change, the custom dice, I will say something. I do not like it! But…but I think it is a great idea which I feel it will be refined and incorporated in future rpgs. For example, the new Star Wars has also a set of custom dice and a similar dice pool mechanic as that of Warhammer 3. It is similar but not identical, and one can see that the designers have corrected “problems” and “errors” found in the Warhammer 3rd edition dice mechanics. This is not a surprise since learning from experience is (should be) natural. Sorry I won’t go into detail but the changes are many, and have to be commented together with other aspects of the Star Wars RPG which have been improved from its predecessor, the Warhammer 3 RPG.