There are a couple of reasons some JW apologists have made the inflated claim that the NWT is the best translation - over and above the fact that it was produced by "God's organisation" of course.
Textual scholar Ernest Colwell produced a list of textual judgements that indicated a Bible translation that followed the best manuscript evidence. At the time the NWT scored best on this scale.
This led some apologists to claim the NWT was the best translation available. But of course this test is about the textual base of the translation not about the quality of the translation itself.
In terms of translation quality, scholar of early Christianity Jason BeDuhn concluded that the NWT was the best compared with other major translations in a book on Bible translation called "Truth in Translation". But he also disagreed with the NWT's use of the divine name in the NT. Others have criticised BeDuhn for focussing almost exclusively on Christological passages and not other controversial aspects of the NWT.