Not really surprised that Khan ranks at the top while Sybok (a lame villain who is dated to the late 60s or a third season bad guy like the Gorgon) seems to be considered the worst.
On the contrary, Sybok is one of the few antagonists whose motivations don't fall apart under scrutiny, and who is not so obviously malevolent that even he should realize it.
Khan and Nero are raving lunatics, whose actions don't conform to any rational decisionmaking. They make good villains because their motivation for revenge could conceivably override proper decisionmaking abilities, but nevertheless they seem incapable of reflective thought.
Shinzon steals a planetkiller and instead of targeting the people who made his life a living hell he decides to attack Earth with it, a world he has never even seen. Seriously, why not destroy Romulus? It's right
there.
Chang collaborates with Starfleet to kill his own chancellor in order to ensure continued cold war with Starfleet. I mean, really? Now, I still rate him pretty high because he was just so damned entertaining, but the Starfleet/Klingon connection was really just there to give Spock something to do.
Rufao collaborates with Starfleet to retake his own planet--a planet populated by 600 people who have no advanced technology. A planet he could settle on with a billion Son'a (if there
are even a billion Son'a) without the Bak'u ever even
knowing about it.
The Borg Queen apparently wants a threeway with Picard and Data, and sacrificed a Borg Cube to get it. What else kept you from going back in time in the Delta Quadrant, lady?
Soran is willing to kill billions to get into paradise because you can't fly a ship into it. However, he only knows the Nexus is paradise because... he flew a ship into it. He should probably try flying a ship into it.
Kruge... well, actually Kruge's motivations and actions are far more rational than anyone else on this list. Kruge wasn't the most charismatic of the villains, but he made sense. All he wanted to secure information on the Federation's new doomsday weapon. Sure, he was a dick, but that's an understandable reason to commit an act of war.
V'Ger and the whale probe don't count.
This leaves Sybok, who apparently received a telepathic message of immense power and sophistication from beyond the Great Barrier, much as Spock did from V'Ger in TMP. So he sought it out, using his own mind-clouding abilities to obtain a starship. And when he found out he had been deceived by the creature within the Great Barrier, he gave his own life so that others might live. He was a great guy whose every action made sense.
He was a better-conceived character than any other film Trek antagonist to come before or since. Trek V has its problems, but Sybok's not why the movie is considered a failure.