With 12 seasons to its name, The Big Bang Theory had time to give every main character plenty of time to shine. As an ensemble cast, the show couldn't have succeeded without all the characters as a unit. Sheldon couldn't have succeeded without Leonard's friendship, Penny wouldn't have found herself without the help of the group, and Leonard became a better man and scientist with his friends a part of his life.
Each episode had important character storylines that were a part of their development but some seasons were also massive for each person's growth. Season 1, for starters, was instrumental for Leonard, whereas season 12 was life-changing for Sheldon. Each season was big for at least one of the characters.
Season 1 - Leonard
Season 1 was all about Leonard and Sheldon's dynamic as best friends, colleagues, and neighbors with Penny. Along the way, Raj and Howard are introduced and a friend group is created. As important as season 1 is for each character, it was all about Leonard.
Not only was it the start of Leonard and Penny's relationship timeline, but it proved that Leonard was the nucleus of their social circle. Sheldon didn't do much without Leonard, and Raj and Howard didn't want to hang out with Sheldon without Leonard. Not to mention Penny related with Leonard the most. With Leonard's help, the group was whole.
Season 2 - Sheldon
In season 2, Sheldon's personality really came out to shine. He showed that he had problems connecting with people because he didn't always understand social cues in 'The Friendship Algorhytem,' his friendship with Penny grew when he lent her money, and he wraps up the season with an opportunity to go to the North Pole.
Sheldon and Penny's friendship grew to be one of the best in the series and it all started in season 2. While his friends had important storylines too, Sheldon's true personality came poked out in this season.
Season 3 - Leonard
Season 3 was another big season for Leonard. After two whole seasons of pining over Penny and wanting to do more with his career, his dreams slowly came true at the start of season 3.
He and Penny were dating, the introduction of his mother rounded out his character background, and oddly enough the downfall of his relationship with Penny proved he had more to offer. Sure, fans couldn't let go of the fact that Leonard told Penny he loved her during coitus, but being dumped by her proved he had the ability to grow and be happy without her.
Season 4 - Penny
Penny has had an amazing transformation over the years but she really came to be in season 4. There, Penny realized that she really was in love with Leonard after all this time and made a mistake breaking up with him. Sadly for her, Leonard was in a happy relationship with Raj's sister, which led Penny to lean on her new best friends, Amy and Bernadette.
It was with the help of her best friends that Penny became more confident and put herself out there. She did have a controversial moment by "sleeping" with Raj at the end of the season, but it was just another learning moment for her character.
Season 5 - Howard
Howard is the underrated star of most seasons, but season 5 was one of his best. In season 5, Howard had the opportunity to go to space and he prepped for it throughout the season. His preparation allowed him to show more of his relationship with his mother and how it affected his relationship with Bernadette.
The two were engaged and got married before his trip to space, which made most of the season all about Howard's career and relationship. Leading up to the launch, viewers saw how much his friends admired Howard contrary to all the jokes they play on each other.
Season 6 - Raj
Raj made season 6 all about him. The season kicked off with Raj opening up about his loneliness and how he wished he had a relationship like the rest of his friends. It was the start of Raj's biggest storyline: a hunt for the right woman or relationship. That wish of his came true when he met Lucy in this season but their relationship didn't last long.
Outside of his on/off relationship with Lucy, Raj created a deeper friendship with Stuart. This showed great depth in his character as he was starting to grow outside of friendship with Howard.
Season 7 - Amy
In season 7, Amy's bond with the group grew. She got to know Howard on a better level in 'The Scavenger Vortex,' she started doing some research in close quarters with Sheldon at Caltech, and she found out she changed Sheldon for the better in 'The Table Polarization.'
In some major ways, Amy and Sheldon's relationship timeline expanded in season 7 when they went away for Valentine's Day. Amy found her voice and told Sheldon that she deserved more when he was ignoring her for a stranger he met on a train. By the end of the season, Amy was an important member of the social circle and a focal point in Sheldon's life.
Season 8 - Leonard
Leonard's independence came out to play in season 8. He and Penny were more serious in season 8 and he was ready to take bigger steps in their relationship, like living together. His conversation with Sheldon in 'The Space Probe Disintegration' proved how much he loved Sheldon (despite his annoyances) but was ready for more with Penny.
It was in this season that Leonard's idea with Sheldon got published and was commended by none other than Stephen Hawking. He was also notable enough to be asked to speak at his alma mater's graduation. It was a big season for Leonard both professionally and personally.
Season 9 - Sheldon
Sheldon had a defining season in season 9. At the end of season 8, Amy broke up with him, meaning Sheldon had some self-discovery to do. It was one of the saddest things watching Sheldon go through the breakup. Instead of wallowing in sadness, he focused on his work. With the help of his friends and his job, he became a more secure person.
Luckily for Sheldon, he and Amy got back together. It forced Sheldon to get outside of his comfort zone and prove to Amy how much he needed her going forward. Viewers also met his "meemaw" and saw more episodes of his YouTube channel, Fun with Flags.
Season 10 - Bernadette
By season 10, Bernadette was an underrated character who had been a part of the friend group for a long time. Here, she was pregnant and preparing for motherhood while also focusing on her career. Bernadette made it clear she wanted to be a working mother and was refreshingly honest about her concerns.
While focusing on work and a new baby, Bernadette also welcomed a new guest into her and Howard's home, Stuart. Seeing Bernadette as a mother to an infant and, basically, Howard, Raj, and Stuart was important because it showed a close dynamic between them all and how important Bernadette was.
Season 11 - Penny
As important as the earlier seasons were for Penny's growth, she came full circle in season 11. By the 11th season, Penny had a successful career where she had the chance to work alongside Bill Gates, she was married to Leonard, and she even found a comet for Raj.
While Penny wasn't always smarter than the guys, she was far more mature and logical by this season. And compared to the others, Penny's focus on her career and friendships with Bernadette and Amy made season 11 all about her.
Season 12 - Sheldon
It's hard to not think of Sheldon when referring back to the 12th and final season. In season 12, Sheldon and Amy were newly married and their theory for Super-Asymmetry won them both a Nobel Prize. His journey to the Nobel took the entire season, and it wasn't until he was on stage that fans realized how much he grew from season 1 to season 12.
He achieved everything he wanted by the final season and became a better friend and partner. No one earned more praise than Sheldon for this season.