Though he might be iconic, Nightwing doesn't have many recognizable villains, which might be explained by the fact one of his best villains only appeared once. Nightwing has fought hard for his independence from Batman, but despite having his own city, Dick Grayson often has to fall back on using Batman's rogue's gallery, which shouldn't be the case, considering in his own series he has had fantastic villains that should come back.
Bludhaven is a town run almost entirely by casinos, which lends itself to corruption and criminal activity. Despite this, Nightwing really never got his own rogues' gallery, at least not with any staying power. There have been attempts to give him unique villains, in the original Chuck Dixon run of Nightwing readers were introduced to characters such as Lady Vic, Brutale, Nitewing, and Torque, but the first two were simple assassins only in it for the money and the later two were not terribly intimidating. Even Blockbuster, arguably the most reliable Nightwing villain was originally a Batman villain, first appearing in Detective Comics. Even the character most people consider to be Nightwing's greatest villain, Deathstroke, is actually more of a general villain or a Titans villain, he's also, ironically, gotten so popular and so large that with his own comic book, he can't really be relegated to just being Nightwing's nemesis. There's a reason Slade doesn't make many appearances in Nightwing's solo titles.
But In Nightwing #35 by Sean Humpries and Bernard Chang the character The Judge is introduced as a villain that Nightwing has encountered many times over his career, first as Robin, then just after his time in the Titans, and a third and unfortunately, final time, as Nightwing. Nightwing is wracked with guilt over the fact he failed to capture The Judge the first two times they met, which allowed The Judge to go on and murder 32 individuals, a number that haunts Dick throughout the story. The Judge is the only original Nightwing villain to be framed as having been there throughout his career, something that The Judge is even aware of, mocking Nightwing over his failure to stop him. This adds the personal dynamic that generic assassin villains, like Lady Vic or Brutale are missing.
Nightwing often spends the majority of his time dealing with general gangster or mafia-type villains, like Blockbuster. The Judge brings a wild card element to Nightwing's rogue's gallery, much like Joker does for Batman. Considering that The Judge's powers are based on the power to compel people to act on the corruption in their hearts, this opens a rather large amount of story elements, The Judge can literally turn any other character in the story into his pawns, which could really push Nightwing's tendency to trust people or believe the best in people.
The Judge's presence also helps ground and provides history to Bludhaven, everyone knows the history of Gotham, with the five founding families, and how the city works. But Bludhaven has very little personal history shown, which is why it's a huge deal that The Judge is revealed to have been Jacob De Witt, one of the original judges for Bludhaven, over 200 years ago. This naturally ingrains Jacob into the city where Nightwing is, providing a very solid foundation for him as a villain, whereas with a villain like Blockbuster, who could just go to another city. The last time Nightwing saw the Judge, he was locked up in prison but hopefully, one day, this villain can make his return.