IMO, they killed the Borg when they added the queen.
I can at least see there being a logical purpose of a queen for a hive mind. Something to bring order to chaos.
The reason why "DARK FRONTIER" defanged the Borg so badly is actually a simple reason, and ultimately why the FRIDAY THE 13TH remake movie sucked, as an example.
The Borg Queen made a deal with Seven instead of just letting the attack happen and assimilating the crew and her. They didn't need to do that because they are FAR more powerful than a single Intrepid class ship. It broke the character of the Borg irreperably. With my other example, Jason Vorhees NEVER TAKES PRISONERS. It went completely against his character, and defanged him completely. When you make an established danger like those two do something comparatively benign from their established nature, you throw all the menace and danger out the window.
(I'm not forgetting about the "SCORPION" deal... in that case, Janeway had actual leverage because the Borg were losing. I still don't agree with her doing the deal, but at least it didn't declaw them like the deal the Queen did with Seven.)
And Janeway's undercover assimilation in the "UNIMATRIX ZERO" two-parter... final nail in the coffin.
I like VOYAGER a lot, but the one thing I can never forgive them for is defanging the Borg.