The Borg Queen tells us in 'First Contact' that they seek perfection. What do they view as perfection? They assimilate every race they think would benefit them. "Your biological and technological distinctiveness will adapt to service us." She tells Data that he is an imperfect being created by imperfect beings so that means all the tech and biologicals they assimilate are imperfect in their opinion. What then is their idea/definition of perfection if they assimilate imperfection? Is there any explanation to this seeming dichotomy?
According to the Queen herself in "Unimatrix Zero":
BORG QUEEN: It's a shame you're not alive to experience disembodiment. It's the epitome of perfection.
I think there are two elements to the Borg vision of perfection. One is collectivization itself. As @ATB points out, the Omega Particle is "perfect" in and of itself. But the reason it is perfect is because it represents "Infinite parts functioning as one." This is an ideal that the Borg seek. The other element of perfection has something to do with being the most "advanced." I have to put "advanced" in quotes because I'm not actually sure what that means.
The Borg work toward perfection by 1) the assimilation of biologically and technologically distinct traits/items, and 2) the eradication of individuality. This is a bummer, but the Borg don't invent, they assimilate.
Regarding technology, perhaps they desire any item or knowledge that increases what the Borg can't already do or know. Regarding "biological distinctiveness," perhaps the Borg desire any trait that serves Borg needs better than what they have now. For example, the Borg need tactical drones and find that Talaxians and Hazari musculature is excellent for that purpose. The Borg pursued Species 8472 because they believed it was the "apex of biological evolution." On the other hand, the Kazon were "unremarkable" and "unworthy" of assimilation.
Once they have assimilated an individual, they use the vinculum to "bring order to chaos." Drones who are separated from the collective talk fondly of having everyone's thoughts in their head. In the Voyager episode "Unity," Chakotay voluntarily joins a mini-collective and describes it as "incredible....I heard all of you, your thoughts, inside my head, as if they were my thoughts. And I could see myself, through your eyes." He and Riley Frazier then proceed to get it on, because--it seems to me--the link provided a simulacrum of love. To know someone intimately, to think and feel and desire the same things is quite compelling to many.
So, (jokingly) Borg perfection could be analogized as this: soul-mates losing themselves in each other in a big love fest of self-congratulatory harmony, then going out and getting more hot bods to join their orgy.
The OP points out that it is ironic for the Borg to pursue perfection and yet do so by assimilating imperfect beings. I think the Borg seek to add "distinctive" traits and technologies, such that if a species has something good about it, they'll take that and conform the rest of the individual with cybernetics and the vinculum. For example, humans are not all that advanced compared to other species the Borg have met previously, but the Borg were intrigued by human indefatigability enough to make a Locutus. The Queen also noted human nobility as "a quality we sometimes lack."
The Omega Molecule.
The Borg learned of the substance in 2145 through the assimilation of thirteen species, and were able to synthesize a single molecule of Omega that remained stable for one-trillionth of a nanosecond. The experiments the Borg conducted on Omega destroyed a total of 29 Borg vessels and 600,000 drones. The Borg, who referred to Omega as Particle 010, regarded Omega with near-reverence as they believed it to exist in a flawless state. Because the Borg saw the Omega molecule as "perfection", all Borg were ordered to assimilate it at any cost. From their data, the Borg designed a harmonic resonance chamber that could theoretically stabilize the molecule, but according to Seven of Nine they didn't have enough boronite left to synthesize more Omega molecules. http://memory-alpha.wikia.com/wiki/Omega_molecule