I maintain that the Borg were only really good as a species once, in "Q Who?" Even Best of Both Worlds, as good of an episode as it is, already started deballing the Borg. Once Maurice Hurley, who came up with the idea, left, the writers could never figure out how to work with a hive mind and instead had to keep coming up with ways to make the Borg a more traditional bad guy. (I go back here to my theory that Michael Piller was overrated as a writer and showrunner.)