Ooops. Yeah Dark Frontier. I was racking my brain to remember it, and obviously it just tried to skip over to the decent one.
You might be right, it certainly does the most to ruin the Borg. I think the introduction of the children might just be up there though. Speaking of which, where the hell were they during all this Unimatrix Zero nonsense?
Amusingly I decided to check Wikipedia about which episodes did feature the Borg, just to check there wasn't really bad ones I'd already banished from my mind. On the Borg page, Unimatrix Zero isn't even mentioned in the character history.