If you want a listing of facts and details about the contents of the episodes, then The Voyager Companion
covered them very well, but it suffered from a lack of behind-the-scenes analysis. The TNG one had a decent mix of episode overviews and background trivia. But Erdmann had a level of ongoing, firsthand access to the production unequalled by any Trek chronicler since Stephen E. Whitfield when he wrote The Making of Star Trek
, and unlike that book it covers the entire run of the series. So it's true that "best" is a matter of opinion, but it is indisputably the most in-depth of the companion books.