From least to most dateable, these are the men and women who gave our core six something to complain about. Note that this list only includes characters that appeared in two or more episodes.
Barry Farber
Engaged to: Rachel
Pros: He's a dentist, so we can assume he makes a decent living.
Cons: Everything else. He cheats on Rachel with Mindy, cheats on Mindy with Rachel, and eventually cheats on Mindy with someone else. (They get a tasteful offscreen divorce.)
Via: NBC
Emily Waltham
Married: Ross
Pros: Genuinely seems to care about Ross. Cool accent.
Cons: A total stick in the mud. She also forbids Ross and Rachel from being friends, which — while totally fair given the circumstances — makes us hate her a lot. Worst marriage ever.
Via: NBC
Carol Willick
Married to: Ross
Pros: Loving wife to Susan, doting mom to Ben.
Cons: Terrible to Ross. It's not Carol's fault she didn't realize she was a lesbian when she and Ross got married, but there's no reason she has to be so nasty to him after the divorce. On the other hand, imagine being married to Ross.
Via: NBC
Janice Goralnik
Dated: Chandler
Pros: Well, she's definitely the most consistent of all the significant others, appearing in all 10 seasons of Friends. She's also well intentioned, so there's that.
Cons: Oh. My. God. That voice is like if Fran Drescher decided to take it up a notch. Janice isn't necessarily a bad girlfriend, but she's seriously annoying, and she always manages to overstay her welcome.
Via: NBC