My understanding is the Borg believe: perfection
will eventually be achieved via the assimilation of more and more technologically and biologically distinctive species.
Every time a new species is absorbed their 'distinctiveness' courses through the entire Collective, changing—and enhancing—them. This process is known to the Borg as bringing an assimilated species from chaos to order (or, perfection). Also, absorbing a species is pleasurable to the Borg.
The Borg queen makes note of this once they assimilate the last outpost of Species 10026 during
Dark Frontier.
QUEEN: Assimilation is complete.
SEVEN: Three hundred thousand individuals have been transformed into drones. Should they be congratulated as well?
QUEEN: They should be. They've left behind their trivial, selfish lives and they've been reborn with a greater purpose. We've delivered them from chaos into order.
SEVEN: Comforting words. Use them next time instead of resistance is futile. You may elicit a few volunteers.
QUEEN: You cling to sarcasm because you are afraid to see the truth. Species one zero zero two six is already adding to our perfection. You can feel their distinctiveness coursing through us, enhancing us. Stop resisting. Take pleasure in this.
It is also implied this unnamed species has resisted the Borg for a long period, proving their distinctiveness, and improving their assimilated value. Keep in mind that species that is to be assimilated does not have a choice, and the Borg must have every single one of them to complete yet another step toward perfection.
QUEEN: . . . If those individuals are allowed to survive, species one zero zero two six will survive and continue to resist us. . . .
Not sure if it's intentional on the part of Maury Hurley and Brannon Braga to reference an oblique Victorian poet, but Matthew Arnold made frequent references to the pursuit of perfection in his work [
example]. He equated the venture to the pursuit of sweetness and light, or an ultimately wasteful and unattainable venture regardless of the immediate benefits or value to the pursuer. However, in Arnold's view, pursuing perfection is the most human of fallacies.
With that in mind, I believe the Borg will never reach perfection, because they have never known, do not know, and will never know what perfection is—even if they happen upon it by accidentally assimilating the entire galactic cluster.