The list is pretty short - maybe Philip Elmer DeWitt, Benedict Evans, Ben Thompson and Horace Dediu - but none of them bring the deep and consistent product insight - they are more focussed on broader reporting, strategic assessments, or disruption/market theory.
Gruber, to the best of my knowledge, is the only writer that consistently tracks all the major apple developments with the quality of writing that he brings to the table.
Yes, he's a partisan, (Siracusa's term for him) - and writes for his audience, but his words are reasonably measured, and always have a reasonably rational basis.
I wouldn't continue following him if that weren't the case - I've dropped dozens of other Apple observers over the year, but Gruber is still on top of his game.