The Hawaii Five-0 reboot lasted for a decade, and introduced plenty of memorable villains with it. Fans certainly remember Wo Fat, who was Steve's ultimate enemy in the series, as well as recurring villains Dr. Madison Gray and Frank Delano, for instance. However, some villains, while only making a single appearance, certainly left their mark given their evil schemes and the crimes they committed, or how they affected the members of Five-0.
Dr. Olivia Victor definitely irked Steve, and Gabriel Waincroft was no picnic for anyone, especially Chin. That said, out of the many, many villains that Five-0 faced, there are some that just stand out more than others.
Kelly
For a teenager, Kelly was a brilliant and terrifying evil genius. Wanting to collect on her father's life insurance policy, she arranges a heinous scheme to pull it off. She targets former criminal Tommy, and manipulates him into a relationship, and further gets him to believe that her father is abusing her.
Believing he's saving her, she further coerces him to kill her father, and later, Tommy himself is killed by the police thanks to Kelly tricking the officers into thinking Tommy was a threat. She was clearly dangerous, and as Kono pointed out, it's a good thing Five-0 stopped her when they did before she could cause more harm.
Seth Tilton
Seth Tilton was obsessed with satanic rituals, and in the Season 3 Halloween episode, he abducted two people, one of which he killed, to complete his ritual. He was clearly dangerous, and as Five-0 discovers, had had issues since he was a kid.
Not only that, but his grandmother, portrayed to be sweet and harmless, was anything but, and clearly shared in her grandson's capability of committing murder, as she killed an officer. This was one Halloween that Five-0 would certainly never forget, confronted with a real-life horror.
Ian Wright
Musician and former Disney star Nick Jonas played a very different character in Ian Wright, a hacker with no remorse for his actions and who was always looking to make big money. He initially popped up on Five-0's radar for unpaid parking tickets, but that was the least of his criminal offenses.
He also later kidnapped Grover's daughter. He met a premature death thanks to Wo Fat, who shot him dead and freed Grover's daughter, sending her off to give a message to Steve.
Dr. Madison Gray
Season 7 introduced serial killer Dr. Madison Gray, who tormented Five-0 and former FBI criminal profiler Alicia Brown. She was clever, intelligent, manipulative, and certainly cunning. She plays mind games with Alicia and torches Steve's truck and disappears.
However, she reappears later on in the season, this time claiming to be Lauren Parker, but of course, no one believes her. Still, it doesn't stop her from convincing Alicia to break her out of jail, or taking her to see Alicia's daughter, who Alicia believed to be dead. Madison ultimately confronts Alicia in her home, wanting to turn her into a killer, and she succeeds, as Alicia fires the gun and reports Madison's murder.
Victor Hesse
Before Wo Fat, there was Victor Hesse (played by James Marsters of Buffy the Vampire Slayer fame). Steve had long been after Victor Hesse and his brother, Anton, from his time in the Navy, according to the series' pilot.
Unfortunately, Victor wished to exchange Anton for Steve's father, whom he was holding hostage in Hawaii while Steve was in South Korea with Anton in the pilot episode. Everything went haywire when both Anton and Steve's father were killed, and thus began Steve's mission of returning to Hawaii, heading the Five-0 task force, and intending to exact revenge on Hesse. Hesse was a memorable arch-nemesis for Steve, and later met a deadly end thanks to Wo Fat.
Wo Fat
Wo Fat was undoubtedly the most memorable villain in Hawaii Five-0's entirety, both in the original series and in the reboot. He was Steve's worst enemy, and he was responsible for all kinds of terrible crimes, including murder, and he even framed Steve for the murder of the Governor of Hawaii in Season 1 of the reboot.
Wo Fat's wife came back to get revenge on Steve for killing her husband in the reboot's final season. He inflicted plenty of pain on Steve, physically and emotionally, and he was certainly evil. Audiences were as glad as Steve when Wo Fat was finally killed, by Steve himself, ending Wo Fat's reign of terror.
Frank Delano
William Baldwin, brother of Alec Baldwin, portrayed former cop Frank Delano; the character was found to be dirty and subsequently fired from the HPD. He then created an organization made up of other dirty cops and participated in all kinds of illegal activities, but he went way too far when he decided to mess with Five-0 personally.
Frank manipulated Chin into freeing him from prison in the Season 2 finale by forcing him to choose between saving his cousin, Kono, or his wife, Malia. Chin tried to save them both, but unfortunately, Malia didn't survive. Chin eventually killed Delano and avenged his wife, though he still had the loss of his wife to work through.
Gabriel Waincroft
Frank Delano was a pretty brutal villain, especially for Chin, but Gabriel was even worse. He murdered Chin's father in cold blood, and he chose the life of a criminal, committing all kinds of terrible acts. He was the brother of Malia, Chin's beloved wife, but he couldn't have been more opposite from his sister.
However, upon getting shot, Gabriel apologizes to Chin for murdering his father, and implores him to look after his daughter, Sara, not long before he dies. Chin does take in Sara and love her like his own, showing that on some level, he did forgive his brother-in-law.
Sang Min
Sang Min made his first appearance in the pilot episode of Hawaii Five-0 as a human trafficker, busted by Five-0 while they were tracking down Victor Hesse. Since then, he turned from villain to ally for the Five-0 task force, offering them information and helping them out from time to time, though he usually has ulterior motives, such as when he helped Chin in prison, and used the opportunity to escape prison himself.
He often hits on Kono, and while cunning and obnoxious, he's also hilarious and has certainly grown on the Five-0 task force.
Dr. Olivia Victor
Dr. Olivia Victor made only one appearance in Season 3, but it was memorable considering just how much she got under Steve's skin, especially since she knew exactly how to push his buttons. From moment one, he was convinced that she was guilty of killing her patient, and he refused to let it go, no matter how many obstacles she threw his way.
Even his team had their doubts at one point. However, the Five-0 team discovered she wasn't running a therapy practice, but a prostitution ring, and Steve was certainly happy to catch her at the airport before she could disappear for good, ultimately being the one to outsmart her instead of the other way around.