Lord Praven, Jedi Knight story, Tatooine/Corellia, Sith Pureblood
Scourge, Jedi Knight story
Aristocra Saganu (you can flirt, but it's over way too quickly), Agent story, Hoth, Chiss
The guy from the Imperial THORN introduction quest in the Rakghoul tunnels
Lord Cytharat (flirts, but over way too quickly as well), Makeb
Honorable mention (don't think a romance would work, some one-sided flirting would be enough):
Darth Marr