There's only one instance where I think that Voyager really weakened the Borg. Dark Frontier
, at the end. Janeway threatens the Queen and she caves in? She should have said, "Death is irrelevant, assimilate them!", instead of letting them get away. Then Paris and/or Seven would have done something that would allow our heroes to escape.
Yes, in Unimatrix zero it's quite silly how the Queen starts blowing up cubes left and right (and that Janeways gets sad because of it!), but that was of their own doing.