It's a rare accomplishment for a TV show to be on air for a decade and a half, but ER is one of the few shows that managed to do so. The medical drama was one of the most successful shows in TV history, and it's characters and storylines had a legitimate cultural impact that viewers and fans still remember today. Because the show was on for such an astonishingly long time, it gave the writers and cast the opportunity to develop fictional relationships with an exceptional amount of complexity, depth, and range. However, like most shows, some of those relationships were massive hits, while others were huge misses. Many of those relationships were memorable, but some were memorable for the right reasons while others were memorable for the wrong ones. So, which of the relationships on this long-running classic were amazing and unforgettable? And which ones do we just wish we could forget?

RELATED: 20 Things Only True Fans Know About The Good Doctor

Worst: Carol Hathaway & Luka Kovac

Way back when ER was a TV juggernaut, George Clooney was undoubtedly its breakout star. It's almost hard to imagine that he just got his start as a TV star, but his movie career owes a lot to the success of ER. So, when George announced that he wanted to leave the show, it was a big surprise. In order to soften the blow, ER brought in new actor Goran Visnjic. They said he wasn't supposed to be an actual replacement for George, but the fact that they immediately hooked his character Luka Kovac up with Doug Ross' longtime love Carol Hathaway would indicate otherwise, and it was an unfortunate cheap trick that just didn't work for the story.

Best: Kerry Weaver & Sandy Lopez

One of the best things about ER was that it tackled so many ideas and social issues that TV had never really confronted before. And, although it might seem downright normal now, there was a time where showing gay characters and relationships on TV was pretty much unheard of. But ER made a bold move introducing Kerry Weaver, a main character who happened to be a lesbian, and they made even bolder moves by introducing her relationship with Sandy Lopez. It was one of the first times mainstream TV featured a main character who was gay at all, and giving her relationship with Sandy the same weight as every other relationship on the show was a landmark achievement.

RELATED: The Myers-Briggs® Personality Types Of Grey's Anatomy Doctors

Worst: Carol Hathaway & John Taglieri

Even if you've seen ER before, you're probably asking yourself, Carol Hathaway and who? It may come as a surprise, but Carol's relationship with John Taglieri was a pretty long story arc that almost ended with them getting married. However, this TV ship unsurprisingly never even set sail. The main problem with the relationship, as well as every other relationship that Carol and Doug had with anyone who weren't each other, was that it was clearly one of the many roadblocks the writers threw in the way of Carol and Doug's relationship. So, while Carol and Tag's relationship was supposed to be portrayed seriously, it was really just a relationship between Carol and not-Doug.

Best: Mark Green & Susan Lewis

During its exceptionally long tenure on-air, ER loved to play with the will they/won't they relationship dynamic. Although their relationship ended on "won't they", the relationship between Mark and Susan was one of the show's best. Dr. Greene and Dr. Lewis spent almost their entire relationship as best friends, but there was always the underlying tension of whether or not things would develop into more. And although both characters proclaimed their love for one another, ultimately, things didn't work out. However, the chemistry between the actors as well as the legitimate friendship between the two makes it one of the best relationships on the show.

Worst: Jing-Mei Chen & Greg Pratt

When we're talking about a relationship on a TV drama, it's normal to expect drama, but the relationship between Greg Pratt and Jing-Mei Chen was downright bizarre sometimes. There wasn't just drama going on between them; it honestly seemed like most of the time they didn't even like each other. Granted, it's not wildly unrealistic to see a relationship deteriorate to the point that people seem to hate each other, but this is TV. There's no reason to drag things out, and it's not like the audience was super invested in their relationship or something. This has got to be one of the worst, simply because these doctors waited so long to rip off the Band-Aid and just end it.

RELATED: Grey's Anatomy: The 10 Hardest Hitting Break-Ups

Best: Luka Kovac & Abby Lockhart

All of the long-running main characters of ER went through some serious journeys throughout the show, and two of the most epic odysseys belonged to Abby Lockhart and Luka Kovac. Luka and Abby were kind of the Doug and Carol of ER's later years; they were on and off and had a ton of ups and downs, but they always seemed to be drawn back to one another. So then, it's no surprise that they wound up together in the end. Individually the characters were kind of a mess, but after years of growth on their own and together, their relationship became one of the best parts of the series.

Worst: Neela Rasgotra & Ray Barnett

Neela and Ray were a pairing that actually had a lot of potential. But, that potential crashed and burned in a pretty literal sense. Ray and Neela had a great friendship and an interesting back and forth, and they actually seemed like the type who might be destined to end up together. However, after getting in a drunken fight with Neela's current honey, Ray gets kicked out of a party, hit by a truck, and loses both of his legs in the process. Although they eventually reconciled and the show ends with the implication they might finally get together, the whole accident situation kind of killed the momentum of the relationship.

RELATED: 10 TV Shows That Were Incredibly Expensive To Make

Best: Mark Greene & Elizabeth Corday

Over the course of his character's development, Dr. Mark Greene wasn't particularly lucky in love. He seemed to try his best, but after a series of failed relationships and almost relationships, he finally managed to work it out. Once he met Dr. Elizabeth Corday, it seemed like everything was coming up roses for the couple. Their relationship was sweet and solid, and it seemed like they were in it for the long haul, which they ultimately were. Sadly, Dr. Greene discovered that he was dying of brain cancer, and his love with Elizabeth wound up being his last. The ER writers definitely loved torturing the audience with this one!

Worst: Susan Lewis & Jon Carter

Why in the world was this ever even a thing? After years of on-screen friendship, John Carter and Susan Lewis took a shot at an actual romantic relationship. But the show didn't really explain why, and we're guessing the writers didn't even know why. It was one of those TV relationships that seemed borne out of utility instead of actual storytelling. The characters weren't in relationships with anyone else, and it was just an easy shortcut to give them both something to do and a way to interact with one another. The problem is, it always really felt like a shortcut.

Best: Doug Ross & Carol Hathaway

I mean, who else could possibly top this list? It's pretty unusual to have a universally loved fictional relationship, but Doug and Carol fit that bill to a T. The whole world knew they were meant to be together, and the show mercifully agreed, even if they dragged out the torturous drama as long as they possibly could. As individuals, Doug and Carol were messes, and their relationship was often times a mess—but they were just meant to be! Heck, even George Clooney thought so. Reportedly, he agreed to make an appearance in Carol's last episode specifically because he always wanted the pair to get back together. Obviously, we feel you, George.

NEXT: 18 Best Doctors In Television History