(Wikipedia article) (Memory-Alpha Article)
I am surprised this wasn't added to this list, and though it's not a super long answer, it appears to be valid by the constraints on the question.
Spoilers of certain episode plot lines exist further on, abort now if you wish to avoid them!
Lieutenant Natasha ("Tasha") Yar, who was security chief (and tactical officer) on the Enterprise D before Worf was assigned as chief of security / tactical after her death in the series. She was a big part of the USS Enterprise-D's crew as chief of security and tactical officer.
And this is a short list of aliens she played, but she played a well known nemesis of the Enterprise and the Federation...
If you didn't know this before, Denise also played Romulan Commander Sela, a well known Romulan enemy.
Sela was behind the following major events, to my best recollection (this is not, however, a complete list of her appearances):
- Geordi LaForge's abduction and brainwashing (in which he tried to kill a Klingon official, during TNG 4x24, "The Mind's Eye"), and
- providing aid, supplies, military assistance, and other resources to the Duras family during the Klingon Civil War (TNG 4x26 and 5x01, "Redemption", parts 1 and 2).
- She was behind the plot to send Vulcan ships filled with Romulan troops to Vulcan, during "Unification" part 2 (TNG 5x08), in an attempt to invade Vulcan under the pretense of 'reunification' of the Vulcan and Romulan peoples.
All images listed here are from Memory Alpha's page on Denise Crosby, though the system here on StackExchange forced an Imgur upload.