Definately the series. The introduction of the Queen to give the Borg a single voice in my opinion really wrecked them. When they take Picard to assimilate him in Best of Both Worlds, and he's standing in the cube trying to talk to the Borg, you really get the impression that this thing is huge, powerful, and doesn't care about any of the same things as us - summed up in the exchange "We would rather die!" "Death is irrelevant. Resistance is futile": Picard has a visible reaction to this, and it's clear that he's fully realising just how different the Borg are (or should it be "is", if it's really just one mind? I dunno
). Then you have First Contact, where the Queen has to resort to seduction to get what she wants and appears to have a slight aversion to dying.
I also didn't get, if the Borg were able to assimilate/"consume" technology as it was shown and stated that they could, why they didn't just assimilate Data instead of going through the whole seduction-and-giving-him-human-skin fiasco. Obviously from a film perspective it makes more sense I suppose, but from an in-universe perpective it makes the Borg look weak and inept - I mean okay, Data was an advanced piece of technology, but he was still made by a Federation scientist and the Borg are shown to be far, far more advanced than the Federation and have presumably encountered life-forms with technology far superior to anything the Federation have - and let's face it, an assimilated Data would have been a far more effective villain for the film than the Queen!
Of course, this is also the Collective that takes the long way round through the Federation fleet when they apparently have a transwarp conduit that opens like, a light-year from Earth
Erm, well that turned into a bit of a rant, but I think my point was - the Borg were scarier when they were ruthless, powerful and unstoppable with no agenda other than to add your biological and technological distinctiveness to their own.