I read three Nancy Drew: Girl Detective books over the weekend (17-19), and I think I can safely say that Year 3 of the series is my favorite year so far! (I'm pretending the bad Book 16 doesn't count as part of Year 3.)
Years 1 and 2 were okay, but they were very inconsistent. The characters change their personalities far too often for my liking. The books that were originally written for a different Nancy Drew series stand out as odd, and it feels like at least four books were rushed through production, to be published in time for the Nancy Drew 75th Anniversary.
Year Three is a distinct improvement over the two previous years. The humongous continuity errors start to disappear, and the characters are more consistent! I'm guessing that this happened, because they rehired authors from earlier books. Or maybe they hired new authors, who were smart and read other books in the series, before writing.
Year Four is when they made the decision to reboot Girl Detective into a trilogy series. And judging from the way that the publishing schedule gets messed up, the strategy was to get rid of all remaining books as soon as possible, to make room for the trilogies. Of course, it's possible that I'm wrong, and the writers were just extra productive that year, but I won't know until I get there, with Book 22.