I wouldn't say they're the perfect enemy, like Moriarty to Sherlock Holmes. The Romulans are that for Star Trek, an empire of equal might & reverse ideologies. However, The Borg are the perfect
horror for our Star Trek heroes. There would be no battle fronts, neutral zones, espionage or enemy tactics with the Borg. They are simply a horror to repel, the worst they've ever known, from their prespective