Although the titular group of Friends was always the focal point, the relationships that the core six had over the course of the series often drove the plot - and there were a lot of relationships over the ten seasons! No matter how long the romance was, they all somehow tested the cast ; be it through unusual yet unignorable flaws (dirty apartments, for example) or returning a season or two later to throw a heavy curveball their way.
Some of these exes were particularly difficult to the point of being unhealthy for the main six. On the other hand, other exes were good people and actually bought out the best in the friends.
Best: Julie
Julie was Ross's first 'serious' girlfriend. Introduced at the very end of Season 1, this is one of the relationships in the show that was a shame to see come to an end. Ross and Julie were a match made in heaven - they had similar interests, had a very healthy and trusting relationship, and all of Ross's friends liked her... except one.
If Rachel hadn't drunkenly confessed her feelings for Ross, Julie (and the cat they never bought) would perhaps have still been with Ross for at least a few more seasons.
Worst: Paolo
Dubbed the 'weenie from Torini' by a pining and jealous Ross, Paolo was Rachel's first boyfriend in the show.
Whilst Rachel was head over heels for him and their relationship seemed healthy (at least in the 'physical' sense), it is very easy to see why Paolo is a terrible ex. The simple fact that he made a move on Phoebe and did not appear sorry for what he had done is more than enough to classify him as one of the worst exes in the show.
Best: David
There is no question that if circumstances were different, Phoebe would have ended up with David. The sweet shy scientist first appeared in Season 1 and the moment he met Phoebe, there was a spark. But because of a research trip to Minsk, Phoebe would only see David two more times throughout the series.
But the times they were together are some of the most tender, heartfelt, and romantic moments in the show. Aside from Mike, Phoebe did not show the same amount of love and adoration for any of her partners as she did for David.
Worst: Tag
The young and handsome Tag was Rachel's assistant in Season 7. While Tag was a good and well-meaning person overall, he unintentionally bought out a very unprofessional, almost cringe-worthy side to Rachel.
She hired him over a more qualified candidate purely for his looks, she lied about his sexuality so other girls would not date him, and even got Joey to make sure that Tag didn't date. Even when they were together, Rachel remained unprofessional and nearly lost her job in one episode because of it.
Best: Carol
Carol is Ross's first wife, and though their relationship is never shown onscreen, it is implied that they were a good match while they were together. But what puts Carol as one of the best exes is her relationship with Ross onscreen.
She and Ross are on great and work well together in raising their son, Ben. Their positive relationship is further cemented when Ross steps up to give Carol away at her wedding to Susan. This is one of Ross's best moments in the series.
Worst: Kathy
Kathy has the rare fact that she is the ex of two of the main six: Joey and Chandler. She has the even rarer fact that she is the ex of two friends and secretly cheated on them both.
This is not the only reason why that she is one of the worst exes in the show. As Joey's girlfriend, Chandler's attraction to Kathy drove him mad, and as a result, she inadvertently threatened Chandler and Joey's friendship. As Chandler's girlfriend, things were going well until a handsome co-worker of Kathy's brought out Chandler's insecurities, and he bitterly and falsely accused her of cheating.
Best: Richard
If Monica and Chandler did not end up together, Monica would most certainly have still been with Richard. Despite the age gap, Richard bought out a lot of good in Monica, as she did for him. They were both very much in love with each other for their personalities and their chemistry was lovely to watch.
Although Chandler and Monica's relationship is undeniably one of the best in the show, it's almost heartbreaking to see Richard realize that he'll never be with the woman he loves.
Worst: Emily
There is no question that fans would Emily as one of the show's worst exes. But she did not start off as such. Before the wedding, Ross and Emily's relationship, while very fast-moving and perhaps doomed from the beginning, was overall a good one.
It is when Ross accidentally says 'Rachel' at the altar that Emily's character seems to take a U-turn and she makes Ross's life absolutely miserable. Her unreasonable demands were undeniably toxic and brought Ross down.
Best: Janice
Janice is perhaps the most famous ex in the show. The on-and-off girlfriend of Chandler, Janice has both entertained fans and annoyed characters with her nasal voice, braying laugh, and distinctive 'Oh. My. GOD.'
Yet despite all these quirks, Janice was a good girlfriend for Chandler. Whenever they were together, she doted upon Chandler, the others liked her (except Joey) and during their relationship in Season 3, she actually made Chandler really happy. This was perhaps the closest Chandler got to a serious and loving relationship before Monica.
Worst: Janice
On the other hand, Janice has a lot of traits that make her one of the worst exes too. For the majority of the time, she irritated and irked Chandler to the point that he would pull silly stunts just to get rid of her (lying about going to Yemen is perhaps the most notable example).
Furthermore, she threw plenty of curveballs later in the show when Chandler was with Monica, such as inviting herself to their wedding. Whether she was good or bad, Janice is certainly an unforgettable ex!