As one of the oldest sci-fi franchises around, Star Trek has created a huge number of memorable characters that became fan favorites. And even though the characters keep themselves busy (working on space ships is no easy task), they still find the time to form romantic relationships. Sometimes they work out in the long run, sometimes they don't.
The truth is that some heroines of the Star Trek franchise are better at relationships and would, therefore, make better girlfriends. It's clear from the shows and the movies that certain characters are more in touch with their feelings or their approach to love makes them a better romantic interest than in other cases.
Best: Deanna Troi
Deanna formed an epic romance with William Riker in the end. However, she could make it work with someone else as well, if both people put the work in. Thanks to her powers as a Half-Betazoid, Deanna can sense the emotions of others so she'd be able to diffuse any emotionally charged situation in a relationship. That would keep the romance with her peaceful. Not everybody wants a big drama, after all.
Worst: Mirror Kira
Most people coming from the Mirror universe turned out to be not so great but the Mirror Kira represents them all well. Since she has zero regard for other people, entering a relationship with her would be highly dangerous, and undoubtedly lead to getting hurt. The original Kira also had her issues but she was a much nicer and caring person than her Mirror counterpart.
Best: Grilka
Even though Grilka didn't play a major part in Star Trek: Deep Space Nine, she still managed to leave an impression. Klingons often don't like to associate with other species but Grilka proved she wasn't prejudiced as she dated Quark. Grilka was also smart and determined and if somebody likes dating active people who know what they want, she'd be perfect for them.
Worst: Lwaxana Troi
Lwaxana Troi, Deanna's mother... was a lot to handle. She insistently flirted with the men she set her eye on and wouldn't take no for an answer. As a result, it was no wonder that other people often find Lwaxana annoying and did their best to avoid her.
Besides Captain Picard, she also tried to romance Odo but in both cases, it didn't work out well for Lwaxana, for understandable reasons.
Best: B'Elanna
B'Elanna and Tom Paris ended up together but even before that happened, it was obvious B'Elanna was great relationship material. She's strong, intelligent, inventive, and overall a good person. She also doesn't let people treat her badly so it's easy to rely on her. B'Elanna was easily one of the more sympathetic Voyager crew members, and this would translate well in the romance department too.
Worst: Vash
As Captain Picard's former girlfriend, Vash was charming but also opportunistic and manipulative. As a result, she sometimes proved too much even for the all-powerful Q who formed a relationship with her. Vash loved adventure and profit and these two things motivated her more than love or affection. As a result, one would never be able to rely on her in a relationship, not really.
Best: Uhura
Uhura might not have had as much space as she deserved in the original Star Trek series - but she was still one of the most sympathetic characters.
Uhura's intelligence, kind nature as well as a no-nonsense personality would make her a great girlfriend. Plus, she had a great singing voice and wasn't afraid to make fun of herself.
Worst: Borg Queen
The Borg Queen is probably one of the worst imaginable girlfriends. As the Borgs form a collective she rules, she doesn't feel and see the world in the same way as other people. She can be manipulative, as shown when she tried to use physical attraction (among other factors) to make Data betray his crew and work alongside her. To start a relationship with her would also mean becoming a part of the Borg collective and that's a sacrifice rarely anyone would be willing to make.
Best: Jadzia Dax
Jadzia was one of the most intelligent, friendly, and funny women in Star Trek: Deep Space Nine. She had lived multiple lives thanks to the symbiont in her body, and as such, she seemed wise beyond her years. However, Jadzia was also a passionate person and wasn't afraid to pursue someone when she wanted him. In her relationship with Worf, she made the first step. Jadzia would be a strong and supportive partner in any relationship.
Worst: Seven Of Nine
Seven of Nine is a beautiful and intelligent woman. But considering her traumatic past and the association with the Borg, she's not the best person to form a relationship with. In one of the weakest Star Trek: Voyager storylines, the series paired Seven with Chakotay but it fell flat. Seven of Nine simply comes along better as a single person fighting alongside her crew and friends.