Oh man, this is difficult.
Liara is pretty much Bioware's "canon" romance I guess. Out of all the character's I think they focus on her romance (or the scenes anyway) the most. While I love what she does for Shepard, I just couldn't see myself with her. Still think she's a very good choice though.
I like Ashley and what she goes though in the games, but her romance itself leaves a lot to be desired. Even with the Citadel DLC it seems a bit bland.
Kaiden is the go to romance for FemSheps I guess. I've never really seen his romance arcs fully (always felt he was a good guy, not romance material. Might have to do with his personality.), but I love his romance scenes, even with MaleShep.
Thane's is heartbreaking, but you pretty much know what you are getting yourself into by choosing him. I thought he was OK. His romance, especially with the recent Citadel DLC though, is too much (in a good way).
Garrus is pretty damn awesome. If I was playing a FemShep, I would choose him without question. MaleSheps are just settled with a bromance, lol. But seriously, Garrus has a bunch of good scenes, and even if you turn him down in ME3, he takes it like a champ.
Jacob...well, the less said about that the better. Dem Abs aint worth it.
Jack is another good choice, but I felt like a got a bulk of her character and what she was simply by talking to her. But I guess out of everyone she changes the most, which helps the romance.
Miranda always felt like a replacement Ashley, only with better romance scenes. Not that its a bad thing.
Steve and Traynor are great characters in themselves, and for anyone holding out for a gay option that isn't Liara/Kaiden, then these two are really good choices.
I personally chose Tali. She was just too damn cute, and the way she and Shepard interact especially in ME3 is really good. Plus if you chose Ashley in ME1, she is totally cool with you going with Tali!
Its hard to say since some of the romance changes are very small for some, with bigger ones for others. Some can be great characters with crappy romances, but others have good romances with crappy characters.
But if I had to pick...probably Tali, Kaiden, or Liara. Maybe Garrus too although I think Shepard eases into these three the easiest and not out of the blue.