This creepy race of organ harvesters made their first appearance in Star Trek Voyager season 1 episode “Phage”. The episode is aptly named, for the phage is the flesh-eating pandemic that has infected the Vidiians, forcing them into kidnapping and harvesting the organs from their victims in order to sustain their race. This has resulted in the Vidiian society being split into two groups, for those that have contracted the phage are shunned by those who have not out of fear of infection.
Upon hearing that Voyager contained a race of alien antagonists that participated in organ harvesting, I was immediately intrigued for that seemed like a pretty scary and rather morbid concept for a family series like Trek.
I, of course, have never seen a Vidiian but have heard that, due to the nature of the phage and their need to harvest organs, their skin resembles that of a patchworked quilt, which is in of itself a very creepy description.
As with most races in Trek, however, the Vidiians are not portrayed as being completely evil but rather a society suffering from an incurable disease, forcing them to perform unspeakable acts to stay alive, turning them into sympathetic villains, and making it so that, while they are undeniably creepy, they are not inherently evil. So the highest they can hope for on my list is the #3 spot.