Now that I emerged from the tunnel of plots... I can think of other stuff now.
After watching most of the episodes more than once, I am ready to pose a question: who do you consider the sickest, most psychotic villain of Five-O?
Here's some I can think of:
Eddie from "I'll Kill 'Em Again": the creep plans every detail just to get under McGarrett's skin. Like he doesn't have anything else better to doHe commits suicide in the end because McGarrett and the team have bettered him.
Walter Stark from "Nightmare in Blue": impersonates a police officer to rape and murder women but he makes the mistake of leaving one of his victims alive. This episode is tough to watch because of the subject matter but I like the fact that the team is out to get Stark leaving no stone unturned including clearing their own police officers and are sympathetic to the victim, as they should be! Stark comes across as the unfeeling predator he is.
Joseph Trinian from "Yesterday Died And Tomorrow Won't Be Born": released after serving 15 years for murder that was committed in 1953, during or just after the Korean War. I know Todd put on a thread that he wonders if Trinian was actually framed. After watching the episode again yesterday (and laughing at the boom mike again!), it seems to me Trinian simply stewed while in Portsmouth. He returned to Hawaii with his revenge plot in mind. Which begs the question, was Trinian stationed there prior to Korea? Why would he go to Hawaii to begin with? And how did he know Swenson was the witness against him since Swenson/Rutgers was living under an assumed name? Regardless, Trinian is as cold-blooded as they come except to his wife Emma. Todd also pointed this out in his post, making Trinian a complicated person and that much more real.
I'll also add Arthur from "Draw Me A Killer": He's lost in his own world with his fascination/obsession with Judy Moon. He takes out four people before Five-O traps him in a pretty ingenious way.
It's hard for me to pick one to be at the top, but I have to put Eddie up there because he's calculating the whole time.
What do you think?