I've said before, The Borg were best when they were like a "slasher movie" bad guy.
Just this silent, unstoppable force.
To me, The Borg were great in "Q Who" and "Best of both worlds", past that they started to humanise them and they lost their edge, they were still borderline respectable until Voyager started dabbling with them, but as soon as we saw Species 8472 brutally destroying them, that was that.
Oddly, the Enterprise Borg Episode (forget the name) was somewhat of a renaissance for The Borg, and the first time they had been a legit threat for years.