Favorite Romance: Liara. It's not even a question, really. Despite having a litany of romance options at my fingertips, Liara's just seems to have the most content associated with it (she seems to be the "creator's pet" of the series). You have a whole DLC in 2 that's just focused on her, you can rekindle your romance in 3 very easily right from the get-go, and she always has plenty of dialogue for almost every situation.

Least Favorite Romance: In 3, Allers is terrible, and I question the "genius" who decided to add that in. She flirts with you to such a point that it seems like she's aggressively coming on to you, and when you call her out on it, she just goes "tee-hee, I was just kidding!" The stunt casting of the role aside, this feels like a character who shoehorned in solely to appeal to 12-year-old boys.

Had I romanced Kelly, she'd probably be in that spot, but her situation is more a decision by committee to not focus that much dev time on her, considering she could be killed off in 2 and didn't have that big of a role beyond the ship's yeoman/stripper.