I like topics like this (but hate the crappy squabbling that fills a page and a half). It allows some of what that original reception was to get glimpsed, what it looked like at the time, when Voyager & DS9 were still on the air in "Dark Frontier"'s case and before the rest of Season 5-7 were known. Sometimes things that would be forgotten turn up. An example- a chunk of Voyager's viewers might have been distracted/preoccupied during "Infinite Regress", particularly during the last 10-15 minutes on original airing. Why? Babylon 5's long-anticipated series finale "Sleeping in Light" aired on cable immediately after Voyager got off.
I saw "Dark Frontier" back in Feb 1999 (BTW, it aired about a week after Pluto became the furthest known planet again (back when it was still considered a planet), taking the position back from Neptune, which held it since 1979. Due to their orbits, Neptune is further than Pluto 20 in every 248 years). Heck, I've seen every Voyager episode but 1 and all of them I saw first-run or via tape/weekend rebroadcast so I have a lot of first-hand memories.
"Dark Frontier" stood out because it was the return of the Borg Queen and the first big Borg confrontation since "Scorpion" and the first real direct confrontation as opposed to scorpion fable-styled opportunism. This was a known enemy. Other big 2-parters/cliffhangers had new (Hirogen, Species 8472) or relatively new (Kazon) enemies... or one that's been previewed (Krenim), which gave it a different feel. It felt like, going into it, it would have to prove its worth in the annals of big Borg stories, up against "Q Who?", "The Best of Both Worlds", First Contact, "Scorpion" (but not vs. "I, Borg", "Descent", where the Borg were more secondary to other things). It had feelings of epicness (Unimatrix One, the transwarp coil, planning to heist the gold bullion train, errr, damaged Borg vessel limping back, the Borg Queen) but also a feeling of smallness, like a midseason, midwinter episode (took a lot of time for the story to get going, to establish all the pieces).
It also plugged in well to Seven's presence on the ship, her experiences since joining (similar to how "Hope and Fear" did). It seemed a little weird at first, the Queen going out of her way to get Seven back until it became clear later in the episodes that it was essentially a duel of queens and collectives, and Seven was a symbolic pawn. To the Borg Queen, she just couldn't fathom why someone would want to leave/stay out of the Collective. It was a sort of heresy and her Borg blinders wouldn't let her understand it. I'm sure it helped for others that Seven got to be the centerpiece of the story, which was the first big 2-parter/epic episode centered on her ("Scorpion" was bigger than her, then her story was the B-plot, the undercurrent to "Hope and Fear", she was part of the ensemble to "Year of Hell" & "The Killing Game"). Made for eye candy. Borgs, babes, action.
I never thought "why is she back?" because I knew a little about beehives/ant colonies and know they replace queens. The Borg Queen was less an individual monarch and more a hierarchical role that another Borg being groomed to be queen would fill. I liked how the Queen acted in "Dark Frontier" far more than in First Contact.
And yes, "Dark Frontier" did feel much more satisfying than "Endgame" and thematically, except for the focus on Seven and not an ensemble effort, it would have made a much better series finale.