The Big Bang Theory was well past its days of being nominated for the Best Comedy at the Emmys but audiences still loved showing up to watch the nerdy Cal-Tech physicists and their beloved gang of misfits on CBS when the series entered its second decade of airtime.
Despite being arguably past its glory days, The Big Bang Theory still continued to put up solid ratings and did well enough for the comedy to get one last season before saying goodbye to the Pasadena physicists.
The Collaboration Contamination., Episode 5 (7.4)
This episode truly showed how dependent the other members of the group had become on one another when Howard and Amy begin working on a project together, leaving Sheldon, Bernadette, and Raj feeling lonely without their partners. The obvious solution (which apparently wasn't obvious to them) was to start hanging out with each other while their significant others were busy, and that's exactly what Raj and Sheldon end up doing (though Sheldon does immediately irritate Raj so not a bullet-proof plan).
The Confidence Erosion, Episode 10 (7.4)
While the bulk of this episode was centered around the premise that Amy and Sheldon couldn't plan a wedding that both of them would enjoy so they both started purposefully planning weddings that the other would hate, the writers decided to get real with the B-story when they had Raj ditch Howard as a friend. It was a common trait throughout the series that Howard would poke fun at his best-friend, but Raj finally decided that he'd had enough of the negative energy that Howard brought into his life and told Howard he needed a break from him.
The Geology Methodology, Episode 7 (7.5)
Sheldon never shied away from displaying his mental superiority to others, but for some reason he has always had it out for the science of geology, making it well known how much he disrespects the topic. However, when geologist Bert asks for Sheldon's help in detecting traces of dark matter in some meteorites, Sheldon finds the work to be incredibly engaging and entertaining. He immediately begins to feel ashamed and embarrased and believes people will think less of him for working in Geology - which leads to Bert kicking him off the project and bringing Leonard on board instead, which immediately makes Sheldon jealous.
The Matrimonial Metric, Episode 12 (7.5)
When Sheldon and Amy got engaged in the first episode of the season, naturally it would mean that the couple would have to pick a maid of honor and a best man. The logical choice for both of them would be Penny and Leonard as it's well known that the four of them are closer to each other while Howard/Bernadette/Raj are their own little group.
But for some reason Sheldon and Amy thought the others would get angry for not being picked and decided to test their friends with assorted matrimonial puzzles designed to see how well they'd perform. Of course, they ultimately pick Leonard and Penny.
The Novelization Correlation, Episode 15 (7.5)
Audiences were introduced to Professor Proton in season 6 when Sheldon and Leonard hired their favorite television host from when they were kids to do science experiments. Sadly, Professor Proton passed away the very next season but was then replaced by none other than Wil Wheaton! Wil ends up asking Amy to be on his show and Sheldon takes offense (because he wanted to be on the show) but then realizes how proud he is of Amy for doing it and promoting science to little girls everywhere. Meanwhile, Leonard writes a novel that has a mean character that Penny thinks is based on her and gets offended... until she realizes the character is actually based on Leonard's mother.
The Comet Polarization, Episode 21 (7.5)
Tensions ran high during this episode between a few of the friends. The gang goes up on the roof of their building to look at stars through Raj's telescope, but Penny spots an object and Raj takes a picture which reveals it to be a comet. It turns out that it's a comet that nobody has ever seen before so they get to claim credit for naming it - and both Penny and Raj believe that they deserve sole credit for 'their' comet. Raj eventually puts his own name on it (for professional reasons) but gets bullied by Penny into including her as well.
The Proposal Proposal, Episode 1 (7.6)
Fans were thrilled when it was revealed at the end of season 8 that Sheldon wanted to propose to Amy- but they were less happy when it took the writers another two years for that particular moment to officially happen thanks to Amy and Sheldon's short break.
That said, the moment finally arrived in the first episode of the eleventh season when Sheldon flies to New Jersey to propose to Amy and she says yes. Not only do they get engaged, but Howard and Bernadette find out that they're expecting their second child. A lot of stuff happened in this opener.
The Sibling Realignment, Episode 23 (7.7)
Sheldon never excelled at getting along with anybody, but he stated several times that he found it impossible to get along with his siblings. Audiences had met his twin sister in season 1, but it took 10 more years for fans to meet Sheldon's big brother, George, due to a feud between the two siblings. But when Sheldon's mother refuses to go to his wedding unless his brother comes, Sheldon flies out to Texas to convince his brother to attend his wedding. The two brothers fight for most of the episode, but eventually have a heart-to-heart at the end where they bury the hatchet.
The Bitcoin Entanglement, Episode 9 (7.8)
This episode almost felt like a classic for The Big Bang Theory and that's appropriate since it spent the entire episode flashing back seven years in the future. The premise revolved around the guys trying to find a drive that they had mined bitcoins on seven years prior with constant flashbacks to what they were doing when they were doing the bitcoin mining. The episode finally ends with Sheldon revealing that he hid the bitcoins on a jump-drive that was hanging on Leonard's key-chain (to teach the guys a lesson for cutting him out of the process). Unfortunately, Leonard then reveals to Sheldon that he lost that key-chain years ago, resulting in the bitcoins being gone for good.
The Bow Tie Asymmetry, Episode 24 (9.0)
It's only fitting that the highest-rated episode of the season was the episode that audiences had been waiting 8 seasons for - Sheldon and Amy's wedding. Despite all of the premarital chaos, the gang manages to keep the madness at a minimum (which is a surprise to everyone). Just before the wedding is about to start, Sheldon and Amy share a heartfelt moment - which turns into a scientific breakthrough as they begin working out the math in the bridal suite (completely forgetting about the wedding). Eventually, Penny comes in and breaks up the fun, but it leads to Amy and Sheldon officially tying the knot (with Mark Hamill officiating).