In 1994, NBC launched a fast-paced medical drama called ER that would capture the hearts and minds of viewers for 15 years. From the twisted romantic relationships to the constant pressures of practicing emergency room medicine, ER touched on subjects as heartwarming as they were taboo.

Between the hospital board politics and the strain of a teaching hospital atmosphere, to the ungodly hours on duty and the moments of triumph and peace that made the stresses of the job worth it, ER had its highs and lows, just like every other long-running television series. Sprawling across fifteen seasons, it's a given that some seasons are much better than others. So, how does each season rank, and what made it memorable enough to earn its place in medical drama history? Let's rank the seasons, from worst to best.

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15. ER Season 14 Suffered on Account of the Writer's Strike

ER Season 14 collage bookended by Maura Tierney and John Stamos

One of the best things about Season 14 was the fact that the show reached the 300 episodes milestone. On account of the writer's strike, however, the season only had 19 episodes. While it may not seem like three or four episodes would impact the series too much, it actually forced the alteration of several ongoing plot threads. By this point in the show's run, it had lost a good deal of steam, and one of the few saving graces for Season 14 was a quick visit from an old favorite: former PA, Jeanie Boulet (Gloria Reuben).

14. ER Season 12 Tried Way Too Hard

ER's Season 12 cast, featuring Goran Visjnic's Dr. Luka Kovak in the center

One of the things that kept fans coming back in the beginning was the character dynamic between Dr. Mark Greene (Anthony Edwards) and Dr. Doug Ross (George Clooney), the working relationship between teacher and student that Dr. Bennet (Eriq LaSalle) shared with John Carter (Noah Wylie) and the on again/off again romance Doug shared with Nurse Carol Hathaway (Julianna Margulies). After all of those characters had left the show, it was difficult to fill their shoes, but they tried by bringing in big-name actors that just didn't resonate with viewers in the same way. Season 12 was a perfect example of trying way too hard with hit-or-miss characters played by John Leguizamo and Kristen Johnston.

13. ER Season 10 Had Some Major Issues

Promotional image for ER Season 10 featuring a bearded Noah Wylie

One of the best things about shows like ER is how quickly viewers become attached to characters, and over the course of its 15-year run there were plenty of fan favorites. There were also memorable villains, like arrogant surgeon, Dr. Romano (Paul McCrane), who went out of his way to make the lives of pretty much everyone around him miserable. In Season 9, the writers almost managed to make him sympathetic when he lost his arm to a helicopter rotor evacuating the hospital during a Monkey Pox outbreak. The Fates determined taking his arm wasn't enough, and in Season 10 he was killed by a totally different helicopter, which was sort of absurd.

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12. ER Season 11 Put Viewers to Sleep

ER season 11 cast with Noah Wylie in the center

It's a small miracle that the show made it to Season 12, given how boring Season 11 was. There were moments of excitement, but the overall plot for the season didn't do much to capture viewer attention for long. Even when Abby Lockhart (Maura Tierney) was kidnaped and Kerry Weaver (Laura Innes) finally met her birth mother, it didn't seem like nearly enough to keep the audience invested in the characters. This season also saw the exit of multiple longstanding cast members, with Dr. Carter returning to Africa permanently and Dr. Corday and Dr. Chen also exiting.

11. Things Started to Sour In ER Season 7

Promotional image for ER Season 7 featuring Anthony Edwards, Noah Wylie and Alex Kingston

One of ER's most memorable characters was Dr. Greene. While there were certainly other characters who left their mark across the series, for the first eight years Dr. Greene was a central figure. While this season marked the start of the ongoing romance between Abby and Goran Visnjic's Dr. Luka Kovac, it was also the beginning of the end for Mark Greene. Diagnosed with terminal cancer, Season 7 sees Greene and Corday expecting a baby when he gets the news. With Corday in the midst of a malpractice suit, Greene hides the news about his brain tumor. Instead, he proposes, and the couple marry, but tragedy is obviously on the horizon.

10. ER Season 13 Was the Beginning of the End

The cast of ER season 13 around the nurse's station of Cook County General Hospital

By the time Season 13 rolled around, there were no originals left to remind viewers why they started watching in the first place. With Kerry Weaver moving on, former paramedic Tony Gates (John Stamos) stepped up to become an intern, but not even the dynamic combo of Visnjic and Stamos was enough to merit much interest. This season launched final regular appearances for Kovac and Dr. Ray Barnett (Shane West), as Kovac returned to Croatia to care for his dying father, and Barnett was in a car accident that resulted in the loss of both legs. By this point, the series had truly lost one of its most compelling factors: the ups and downs in mostly positive relationships under the stress and pressures that come with saving lives.

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9. ER Season 15 Had Its High Points

John Stamos in the center of ER's Season 15 cast

ER managed to thrive despite the obstacles it faced. Even with multiple key characters moving on, the show continued to draw viewers in on a weekly basis right up until the end. One of the great things about Season 15 was revisiting important people who played a major role in the shaping of County General Hospital. There were guest appearances by a number of fan favorites, including George Clooney, Julianna Margulies, Alex Kingston, Eriq LaSalle, Noah Wylie and Sherry Stringfield. While fans were sad to see it end, it left viewers with the feeling that even without weekly updates of the drama and excitement, County General is still doing what it does best: saving lives.

8. Doctor Carter took the Spotlight in ER Season 9

The cast of ER Season 9 with Noah Wiley in focus on the left

Everyone struggled with Dr. Greene's death in this season, but life in the emergency room went on. This season saw Corday depart temporarily for England with her daughter, while Dr. Romano suffered a debilitating accident on the helipad that put his career in jeopardy and made him even more mean-spirited than usual. With Kerry Weaver finally in position as Chief of Emergency Medicine and John Carter becoming the spotlight character in place of Greene, the season had plenty of ups and downs that hinted at the beginning of the end, which would take six more seasons to arrive.

7. ER Season 8 Said Farewell to Several Icons

Collage of characters from ER season 8 with Anthony Edwards' Dr. Green in the center

For a lot of viewers, Season 8 was where they started to lose interest because until that point they had been able to rely on Dr. Greene's steadfast devotion and Doctor Benton's unwavering surgical hands. Both Benton and Greene departed the show, with Greene dying from Stage IV cancer and Benton forced to take a position with better hours so that he could fight for his son, Reese. Of course, people don't stop needing emergency medical attention simply because certain doctors are gone. The show must go on, but it struggled to maintain the same level after the departure of staple characters.

6. ER Season 5 Saw the Departure of a Fan Favorite

ER's George Clooney as Dr. Doug Ross standing in front of Nurse Carol Hathaway

Despite his often troubling practices and behaviors, pediatrician Doug Ross was County General's biggest heartthrob, rebel and hero. Over the first five seasons of the show, fans watched Doug sabotage his career, his personal life and his relationship with Nurse Hathaway a number of times. He didn't always bounce back, and in Season 5 he did something morally questionable when he helped a grieving mother kill her terminally ill son. Doug stood by his choice, even though he knew it would cost him everything. After he was forced to resign, he left the show, much to the disappointment of viewers, but he did return periodically for special episodes.

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5. ER Season 6 Brought New Series Regulars into the Mix

Promotional image of the ER staff for Season 6

While there were always new faces popping in and out of County General, Season 6 introduced several new characters who would go on to become series regulars: Abby Lockhart, Dr. Kovac, Dr. Finch (Michael Michele) and Dr. Dave Mallucci (Erik Palladino). Dr. Chen was also back in the ER, where she would stay through Season 11. Nurse Hathaway's departure, however, lands this season a little lower on the list. This season also sees medical student Lucy Knight (Kellie Martin) and Carter terrorized by a psychotic patient, who brutally stabs both, killing Lucy. Carter then becomes addicted to pain meds and ends the season on his way to rehab.

4. ER Season 1 Reeled Everyone In

The staff of ER outside Cook County General Hospital.

The introductory season of ER managed to balance the highs and lows with the high pressure and stress of day-to-day life in a teaching hospital's emergency room. It touched on new beginnings, like John Carter's first day as a medical student and the high standards he would face. Carol Hathaway's attempted suicide brought awareness to the mental health crisis people were still afraid to talk about. Mark Greene's struggle to balance the work he loved with his family life touched on issues many families faced, making it easy to identify with his character. Season 1 was just the beginning of something incredible, and viewers seemed to understand that.

3. Unrequited Love Made ER Season 3 Memorable

The ER doctors and nurses with the surgeons in the background for Season 3

Obviously, there were far more important things going on in the emergency room than romances, but they managed to play a major role throughout the course of the series. Early on, Drs. Greene and Lewis always seemed to be just a breath away from falling in love. It took Greene three seasons to recognize what he was feeling, and by then it was too late because Lewis left Chicago. This season also saw Ross and Hathaway drawing ever closer, the season ending with a kiss. Dr. Benton briefly dated a woman named Carla, who changed his life forever by giving birth to his son, Reese.

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2. ER Season 2 Got Even More Personal

The ER cast transporting a patient from ambulance to emergency room.

When Season 2 rolled around, viewers were fully invested in the characters they'd grown to love. As Dr. Greene faced a cheating spouse and inevitable divorce, Dr. Lewis found herself juggling unexpected motherhood with her job after her troubled sister left her daughter with Susan. This season got into the nitty-gritty with Dr. Ross, revealing his difficult childhood and strained relationship with his father. On the brink of losing his job over breaking protocol, Ross redeemed himself by rescuing a child trapped in a storm drain in one of ER's most memorable episodes. Season 2 also introduced the trials of HIV, diagnosing nurse Jeanie Boulet with the virus on account of her husband's extramarital affairs.

1. ER Season 4 Stands Above the Rest

ER Season 4 cast standing together in the emergency room

Two new doctors, Anna Del Amico (Maria Bello) and Dr. Corday joined the cast for Season 4. Ross proposed to Hathaway, something fans had been hoping for from the start, and Benton began dating Corday, much to the disappointment of his family. Hathaway also opened a free clinic with help from Carter's grandmother, while Weaver learned to better manage the ER. Corday's presence introduces one of the series' most despicable villains: Dr. Romano. This season also brought closure and heartache to Ross, whose missing father was found dead. This plot arc set Ross up for departure in Season 5. When his own friend, Dr. Greene, worked against his attempts to get the position of pediatric attending, it launched Ross into a reevaluation of himself that made Season 4 the most epic season across the show's 15-year span.