How I Met Your Mother gave audiences a decade of hysterical television that culminated with a final episode that did a phenomenal job at splitting the series' fan-base.
Regardless of how the final episode played out, the previous eight season finales that the show produced were some of the best episodes in the series and they always did a fantastic job of leaving the audience dangling from a cliff and making them dread the four-month wait they'd have to endure before receiving their answers.
Last Forever, Part 2 / Season 9 (5.5)
The score says it all. Easily the lowest-rated episode in the entire series, the finale tore at the heartstrings of the shows' loyal fan-base when they decided to kill off the mother that they had spent nine years introducing.
The series ends with Ted concluding his narrative, and then having his kids help him realize that the story has actually been about how he's still loved Robin this entire time. In the long run, the premise makes sense as to why Ted would be sharing this incredibly long tale with his kids... but come on, nobody wanted Tracey to die.
Doppelgangers / Season 5 (8.5)
The season 5 finale had the next lowest-rating with the episode revolving around the fact that Lily and Marshall had made an agreement to start getting pregnant when they had seen all 5 of the groups "doppelgangers". Lily and Marshall think they spot the final doppelganger, only for it to be revealed that the man is actually just Barney in disguise.
Meanwhile, Robin gets offered a job in Chicago and turns it down because of her boyfriend, Don... who then breaks up with Robin when he gets offered the exact same job. The episode ends on a positive note when Lily thinks she spots the final doppelganger and she and Marshall officially try to start having a baby.
Challenge Accepted / Season 6 (8.5)
For the entirety of season 6, Ted had been tasked with the project of deg GNB's new headquarters, a skyscraper that would appear in the Manhattan skyline. In the season finale, just before the building is about to be unveiled, Ted is given another task of picking out the light-bulbs for the building - 50,000 of them.
The vast number causes Ted to panic, overreact and try to get back with his former girlfriend simply because he's afraid of the future and the past is comfy and familiar. But luckily Barney and Robin are able to convince him that the past is never a place to run to and Ted confronts his fears. Meanwhile, Lily and Marshall, after a year of trying, find out that they're pregnant with their first child.
Miracles / Season 3 (8.7)
Season 3 ended with two enormous cliffhangers as the episode dealt with Ted breaking up with his serious girlfriend, Stella, then getting into a car accident and realizing that he loved her and wanted her back. Once he gets out of the hospital, Ted finds Stella and gives her a toy kangaroo... which is his version of an engagement ring as he asks Stella to marry him.
The second cliffhanger was that Barney, after also being in a car accident and shattering practically every bone in his body, subtly revealed that he was interested in Robin (which was confirmed in the next season).
Something Blue / Season 2 (8.8)
Despite all of the complications and problems that Lily and Marshall faced on their wedding day, the reception during the season finale was much more focused on Ted and Robin as they recalled a story about their relationship to Barney (after he pestered them into sharing).
The narrative takes multiple twists and turns but unfortunately ends with the sad revelation that Ted and Robin had broken up 2 weeks prior to the wedding and had been keeping it a secret for Lily and Marshall's sake. The episode ends with them parting as friends and Ted itting to Barney that he's ready to be his wing-man again.
The Leap / Season 4 (8.8)
Ted had been trying to make it as an independent architect since he had gotten fired from his firm and he was spending his entire 31st birthday trying to design a building shaped like a cowboy-hat... and got nowhere. Meanwhile, all of his friends tried to get him to come up to the roof for a party but kept failing in their efforts which eventually resulted in Ted getting beaten up by a goat that Lily had rescued from being slaughtered earlier that day.
On top of that, Ted doesn't get the job for the cowboy-hat building and feels like a complete failure... that is until he takes a job as a professor of architecture at Columbia University and reveals it was the best job he ever had.
Something New / Season 8 (8.8)
The season 8 finale dealt with the gang's preparations for Barney and Robin's wedding, Lily and Marshall's year-long trip to Italy, and Ted's move to Chicago. While Ted explains to Lily why he has to leave New York City (it's because of Robin), Marshall flies out to Minnesota so that his mother can see baby Marvin one last time before they go to Rome.
Meanwhile, Barney and Robin spend the episode feuding with a couple that they encounter in a restaurant, and somehow manage to get the couple to end up engaged despite trying to ruin their relationship.
The Magician's Code. Part 2 / Season 7 (8.9)
Just one season finale prior to Barney and Robin preparing for their wedding, Barney was proposing to a completely different woman. At the end of season 7, Barney and his girlfriend, Quinn, get stopped going through airport security and are eventually held in a containment room due to their lack of cooperation (mostly Barney).
However, it's then revealed that the entire encounter was a trap by Barney and he was just using the moment to spontaneously propose to Quinn - and she says yes! Meanwhile, Lily and Marshall deal with the aftermath of the birth of their son, Marvin, and Robin tries to help a discouraged Ted find love by telling him to go after his old girlfriend, Victoria... Ted does so, and it ends up making Victoria run away on her wedding day for him.
Come On / Season 1 (8.9)
The very first season finale had Ted do the impossible - he made it rain. Ted had been relentlessly trying to get Robin to date him for the entire season and episode after episode he continued to fail. Finally, during the finale, Robin its she's going on a company camping trip and that she might hook up with one of her co-workers if she goes... so Ted does the only thing he can possibly do to stop the trip from happening - he performs an authentic Native-American rain-dance.
The dance works, Robin's trip gets canceled, and Ted and Robin officially begin their relationship. Sadly, the episode is hindered when it's unveiled that Marshall and Lily broke up at the same moment Ted and Robin were getting together (but everybody knows how those two relationships turned out so all's well that ends well).