In twelve seasons, The Big Bang Theory went from a show about two roommates and their new neighbor across the hall, with lots of geek culture references in their interactions, to one of the most loved sitcoms with a beautiful lesson about friendship. As one of the longest-running sitcoms on TV, the show was fairly successful, based on the average IMDb ratings. Overall, each season averaged between 7 and 8, which is quite impressive for a show with so many stories to tell about a pretty girl from Omaha with big dreams and how she became best friends with Leonard, Sheldon, and the rest of their nerdy crew.
Both critics' and audience' reviews for the show were mostly positive and with 77 wins and 253 nominations in total, according to IMDb, it's easy to see why. The show took the tried and tested sitcom formula and put a unique spin on it. So while some seasons felt like a rehash of the last, there are some special ones that helped the show gain traction, and the IMDb ratings for each highlight this perfectly.
Season 11 - 7.3
Although TBBT usually had storylines that lasted a single episode, there were some that carried on for an entire season or most of the show. For season 11 it was Amy and Sheldon's wedding, which took them the entire season to plan.
The rest of the season had sub-plots about Bernadette and Howard expecting their second child, as well as Raj and Howard's brief, but long overdue, spat over Howard's bullying tendencies. Those random storylines and the long wedding planning made the season drag, so it's no wonder every other episode wasn't as intriguing as the payoff of the wedding in "The Bow Tie Asymmetry," which became the highest-rated of the season (9.0).
Season 10 - 7.4
The best highlights of season 10 were Amy and Sheldon's relationship moving up a notch when they moved in together and when he proposed in "The Long Distance Dissonance," which was the highest-rated of the season (8.6). It was a big move for them after their relationship had moved so slowly for the first couple of seasons.
As Amy and Sheldon were one of the most loved couples by the fandom, the proposal drew in a more favorable rating in a season otherwise filled with mundane events. Up until that point, nothing new had happened that hadn't already occurred in the show and their engagement was a much-needed breath of fresh air from Leonard and Penny's up and downs, plus Howard and Bernadette's usual squabbles over money.
Season 8 - 7.5
Season 8 had a lot of big moments, from Amy and Sheldon's first "I love you" exchange to the tragic death of Mrs. Wolowitz. Although the show was primarily a comedy, there were occasionally some heartbreaking moments in TBBT , like her passing.
The rest of the season focused on the group's individual careers with Penny starting as a pharmaceutical sales rep, Sheldon becoming a Junior Professor and later working with Raj on dark matter, and many more. Although the show always had mentions of Science, in this season it felt heavier and too technical, making it a difficult season to enjoy.
Season 12 - 7.5
The final season tied with the eighth, further reinforcing that the longer the show ran, the more the quality declined (at least, according to IMDb ratings). By the final season, most major storylines had wrapped up. The couples in the main group had all settled down and even Raj was exploring the possibility of an arranged marriage to Anu.
There wasn't really anywhere else for the story to go and the show became a collection of overused tropes TBBT used from the first season. The one unique storyline that kept fans coming back was Sheldon and Amy's work on Super Asymmetry that resulted in a Nobel Prize win in the memorable series finale, "The Stockholm Syndrome," that's fittingly ranked highest of all episodes at 9.5.
Season 9 - 7.6
In season 9, Penny and Leonard were finally married, but that was by no means the end of their story. Howard and Bernadette were expecting their first child, which Raj was overly excited about, and Amy and Sheldon slowly worked their way back to each other over the season.
Out of all the seasons, this one focused on their personal relationships, which may explain why it ranks a little higher. It kept fans on their toes, even after the happily ever afters, with Leonard and Penny's fight after their wedding while he was away and Sheldon's failed attempt to move on until he got back together with Amy.
Season 7 - 7.9
The seventh season of the show is memorable for many reasons; from Penny's struggles with her acting career to Sheldon and Amy's first romantic kiss on a train, and Raj's first time with Emily.
The big changes in this season made it more intriguing, including Penny quitting acting and getting engaged to Leonard, followed by Sheldon's freak out over the changes happening and running away on a train. It was one of the few seasons where the show broke formula and everything was up in the air. There were lots of nice surprises too, especially for fans of Leonard and Penny's relationship, plus the fan-favorite Raj who finally found love.
Season 6 - 8.0
As the mid-point of the show, it's rather fitting that the sixth season ranks in the middle as well. By season 6, the show had an established fan base with some coming back for something familiar while others dropped off because of repetitive storylines, such as Raj's woes with love.
Season 6 also introduced a fresh face with Alex and her dynamic with Sheldon as his assistant was one of the funniest parts of the season. In fact, an episode focusing on the two of them, "The Egg Salad Equivalency" was one of the highest-rated (8.5).
Season 5 - 8.0
Season 5 had everything that fans loved about TBBT and some fresh storylines and faces too. Stephen Hawking guest-starred in "The Hawking Excitation," which was one of the best of the season (8.6) and his presence brought much excitement to the show.
This was also the season when Howard and Bernadette got married before he went to space. As far as storylines go, these were some unique ones that drew more viewers in.
Season 4 - 8.1
In this season, Leonard and Priya were officially dating and Howard and Bernadette got engaged. The rest of the season revolved around these couples with the revelation of Raj's crush on Bernadette and Penny trying to find her place in the group, now that Leonard was dating someone else.
It's a little surprising that this season ranked so high, considering it featured a couple that TBBT fans believed was doomed. Regardless, it was interesting watching Leonard fall in love with someone else, even though that didn't last very long.
Season 1 - 8.2
The first season has an impressive rating but still falls short of being the overall best. It's also the shortest with 17 episodes, which might have contributed to its ranking as there wasn't much room for boring, filler content which often bogs down a season.
As the introduction, the show was still finding its feet with this season and slowly gaining fans, so it's no surprise that while it might have a good rating, it didn't top the list.
Season 2 - 8.3
By season 2, Penny, Sheldon, Leonard, Howard, and Raj were established names and fans looked forward to seeing more of them. The highlight of the season was Sheldon's relationship with Penny, especially their first hug in "The Bath Item Gift Hypothesis," rated an impressive 9.2.
Friendship was at the core of TBBT and Penny and Sheldon had the best one on the show. It's only fitting the episode with this heartwarming moment was the best of the season.
Season 3 - 8.3
By season 3, the characters had well-established personalities and quirks, so it was easy to relate to them. While the first two seasons primarily focused on Penny, Sheldon, Leonard, Howard, and Raj, the third finally introduced some new recurring characters with Bernadette and Amy.
Apart from the new faces and the promising storylines they brought on, the third season took a lot of what made the first one great and improved it, making an even better, refined story, with well-defined characters.