It would depend on what you want to test. I used a Robin which worked fine in domestic, but found it would not do 5X test on 300ma RCD's neither would it allow the setting required when using set-able RCD's found on many generators. There were other Robin models which would do what I required though.
So both the Make and Model would largely depend on what you do. Also local suppliers no point in getting a unit which can't be calibrated local.
And of course no need for ExII rated unless working in those areas. So no such thing as best RCD tester only suitable RCD tester for what you require.