For me, the weakening of the Borg as villains began with TNG's "I, Borg".
I loved that episode. Making Picard look Hugh in the eye and decide if he could still kill him made for gripping viewing.
Now if you want to argue about Descent, then I'm with you. They just shouldn't have bothered turning up for work that day. It was subsequently ignored by FC and VOY, which was definitely the right decision.
VOY went to the well too many times as well. Scorpion was excellent, but making a main character a Borg meant they would use them as a crutch a bit too often. Dark Frontier had good parts to it, and was produced very well, but ultimately made the Borg weak. Unimatrix Zero was the pinnacle of this mess with easily reversible assimilation.