It is easy to guess which episode of The Big Bang Theory season 1 fared the best with viewers. The show was still proving itself and had no built-in fan base yet, so it makes sense given viewers' curiosity that the pilot earned season 1's biggest ratings. The Big Bang Theory's pilot did very well in terms of audience share, with 9.5 million viewers tuning into the first episode.
In The Big Bang Theory season 2, the series started to come into its own. Accordingly, the ratings of the sitcom began to improve. Where many shows struggle to outdo the ratings received by their pilots, The Big Bang Theory season 2, episode 15, "The Maternal Capacitance," out-earned its season 1 predecessor handsomely with 12.7 million viewers. It is easy to see why this episode proved a hit, as it is the one where Penny and Leonard almost hook up thanks to Leonard's mother.
The Big Bang Theory season 3 saw the show's popularity soar as episode 17, "The Precious Fragmentation," racked up 16.3 million viewers. Interestingly, this outing wasn't an episode that focused heavily on serious character drama, instead revolving around a goofy plot wherein the friend group fights over a prop ring from the Lord of the Rings movies. While it is not an obvious standout episode, The Big Bang Theory's Sheldon makes this one a classic thanks to his transformation into a Gollum parody.
There are two major reasons why The Big Bang Theory season 4, episode 1, "The Robotic Manipulation," was a big ratings success. The outing was the show's season premiere, which always helps, and it also saw Penny follow Sheldon on his first real date. This hilarious premise guaranteed that the episode would be a classic, and 14 million viewers clearly agreed.
With 16.5 million viewers, season 5, episode 15, "The Friendship Contraction," was a huge hit for the sitcom. Surprisingly, this episode's popularity wasn't down to a Leonard/Penny plot line. Instead, its plot focuses on Leonard and Sheldon's brief friendship breakup. Thus, this episode's success helped prove that Young Sheldon could focus on the show's hero without necessarily needing his original co-stars to return.
A cool 20 million viewers tuned into season 6, episode 13, "The Bakersfield Expedition," and this episode once again proved that relationship drama wasn't the only thing bringing fans back to The Big Bang Theory. A Comic Con-centric outing, "The Bakersfield Expedition" saw the gang get stranded while Penny, Bernadette, and Amy struggled to understand the appeal of comic books in their absence.
A staggering 20.4 million viewers watched season 7, episode 2, "The Deception Verification." This episode centered on a classic battle of wills between Sheldon and Leonard when Sheldon realized that his roommate had returned early to spend time with Penny and had given him the slip. The premise was thin but clearly comedically effective.
While season 8, episode 2, "The Junior Professor Solution," couldn't replicate the success of season 7's biggest episode, the outing was watched by 18.3 million viewers. It is easy to see why, as the episode saw Sheldon teaching Howard while Penny and Bernadette feuded. The ensuing drama proved both these underutilized pairs could produce comedy gold together.
The season 9 premiere, "The Matrimonial Momentum," saw the sitcom focus on one of its most dramatic plot lines ever. When Penny learned that Leonard kissed another woman, she started to seriously reconsider their impending wedding. This story soon petered out, but it was still surprising enough to command an impressive 18.2 million viewers.
In season 10, episode 12, "The Holiday Summation," Sheldon and Amy recounted their trip to Texas while Leonard and Penny struggled to find a Christmas tree. This Christmas episode wasn't anything too far out of the ordinary, but it was a rare occasion where The Big Bang Theory's focus shifted with a How I Met Your Mother-style gimmick where the storytelling was split between the show's heroes. This earned the episode a sizable 16.8 million viewers.
The Big Bang Theory season 11 premiere featured no shortage of drama as Sheldon asked Amy to marry him while Howard and Bernadette came to terms with the news that they were expecting another child. While this all sounds fairly heavy, the sitcom kept things light in an episode that earned itself 17.7 million viewers. While funny, the outing was criticized thanks to Sheldon's predictable behavior and tired persona.
While The Big Bang Theory's ratings remained high throughout its final season, the two-part season finale had by far the biggest audience share of season 12. Episodes 23 and 24, "The Change Constant" and "The Stockholm Syndrome," both earned 18.5 million viewers as fans tuned in to watch Sheldon, Leonard, Amy, Bernadette, Raj, Penny, and Howard hang out together for the last time. While the show wasn't perfect, The Big Bang Theory series finale was still unsurprisingly a major hit. Like the rest of The Big Bang Theory's episodes, the outing's ratings prove just how perennially popular the show was with audiences.