Romulans

The Romulans are one of the most prominent villains of Star Trek, one of the few alien races in the series that has remained hostile towards the Federation, having never truly been defeated nor allied.

The Romulans have a striking resemblence to Vulcans (due to the fact they are a Vulcan subspecies) but embrace a power-hungry philosophy centred on military aggression and imperialism, they have been behind the scenes in many of Star Trek's more prominent conflicts and their animosity with the Federation has lasted generations.

It is worth noting that although the Romulans are one of the few central alien races in Star Trek to have never been allied with the Federation they are far less destructive than antagonists such as the Borg - Romulans also play a large role in recent updates to Star Trek Online, which is set in an alternate-universe from the mainstream Star Trek titles.