Leaving out anyone who was a series regular for the sake of making it more interesting...
Rachel Garrett for sure. A tragic figure who knew what needed to be done and was willing to do it, even knowing it meant all-but-certain death for her and her crew.
I liked Erika Hernandez from Enterprise and wouldn't have minded seeing more of her.
I kind of liked Captain Keogh of the Odyssey (DS9), though he didn't get much screen time before the Jem'Hadar blew him up.
Leaving canon, I'm a fan of Peter David's version of Captain Taggert of the Repulse.