My favorite romances are
1. Ashley Williams - I like tough girls far more than girly girls. I like that though girl attitude. Also she is pretty. Her prejudices against aliens is only superficial. I like her loyalty towards her friends. Also she seems trustworthy to do what is right. Lastly, she seems all the right pieces from all the other romances. For example, she has the controlled toughness of Jack. The natural sexiness of Miranda. Girly side like Kelly, and the strong sense of family akin to Tali and her father.
2. Kelly Chambers - I like Kelly, she is pretty innocent, adventurous,always trying to help others. I would have liked her to be less of a girly girl, though. I would like to find someone like Kelly in real life if I couldn't find an "Ashley Williams".
3. Jack - I find crazy girls hot too. Also, I like Jack because she doesn't conform to what is pretty or normal is. She is herself even though her character mostly a shield. it is just a shield. She just needs someone to understand her. Understanding her makes her more interesting.
4. Miranda - I don't like her, somewhat because of her condescending attitude, toward others. I like her soft side though. That puts her at the bottom of my list.
Honorable mention - Tali - I liked her character in Mass Effect 1. She seemed more "real" as a character. In ME2 and ME3 ,her romance felt she was just a fan of Shep. Sort of how we like celebrities in the media. You can tell it was fan service mostly. In ME3 it was better though, though it felt like high school "boyfriend and girlfriend" type of relationship.Too bad, I would have like to have seen a real romance.
Liara - Even though I have a romance Liara romance save imported from ME1 -> ME2--> ME3 have I couldn't muster the energy to beat ME3 . Overall , she looks good, but she is boring as a character, which makes me uninterested in her. Overall my least favorite, I am not saying she is bad. but I am saying boring as heck.