Every hero needs his nemesis, every goody needs his baddy and although in the Trekverse there is little in the way of arch-enemies, there are a few who crop up more than once.
Star Trek as a franchise though is built more around evolving storylines than your average superhero movies, which tend to bring back in the same opponents time and again, or even the likes of
Doctor Who, where you cant move without bumping into a shipfull of daleks or a Cyberman army hidden away.
Nevertheless, when
Star Trek has bad guys, or girls, they're usually pretty damn good. So to speak.
Kai Winn Adami, played by Louise Fletcher
Originally a lowly vedek, Winn ascended to power after the death of Kai Opaka and by manipulating a series of half-truths that discredited the other candidate. Once in power (and even before she gains the seat) Winn proves to be an arrogant, militant leader who wants nothing to do with the Federation. She is jealous that an alien (Sisko) has received visions from her Prophets and become their emissary, a situation that leads her into constant conflict with Sisko, both as the Emissary and as commander and later captain of Deep Space 9. In her desire for power she unwittingly allies herself with a Cardassian-funded separatist movement, and later with the Cardassians themselves, through the disguise effected by Gul Dukat. She is instrumental in his almost taking power in Bajor and releasing the evil spirits known as the
Pah-Wraiths, but eventually atones somewhat for her misdeeds by trying to stop Dukat, before he disintegrates her.