This is a hard question. I think Dowding was the most important defensive air commander in the RAF, but if he had been asked to undertake offensive operations I doubt he would have done as well.
In my opinion, the best offensive air commander in the RAF was another New Zealander, Arthur Coningham.
Harris was a mixed bag. His 1000 bomber raids were strokes of brilliance, saving the RAF at a time it felt it was on the verge of defeat. His raids against Hamburg and the Ruhr were devastating....but his decision to press on to Berlin smacked of meglomania and poor judgement
Best is not necessarily iconic, but for me theyr are the same....not all that interested in image and PR