Mid-TNG run: Borg (Back when they were scary and cool)
Post-Voyager: Cybermen (After the Borg were neutered)
Well, that was series-specific neutering.
A threat like the Borg requires a lot of cannon fodder to be killed off to maintain their threat while allowing the good guys to get away (like how Doctor Who has the Daleks kill tons of folk to maintain their menace even though the Doctor wins in the end). Voyager couldn't supply any cannon fodder and had to win (otherwise the show would be over) so the Borg had to be easier to beat.
Show the Borg in a series where there's lots of cannon fodder they'll be back to normal.