Leonard Hofstadter had the difficult task of being the straight man in The Big Bang Theory. He wasn't as eccentric as Sheldon, as outrageous as Howard, or as quirky as Raj. And while this helped the character come across as the normal one to outsiders, it also deprived him of some of the show's more uproarious jokes.
Still, and thanks to Johnny Galecki's gift for comedy, Leonard held his own, outshining the other three numerous times. Most of his jokes were made at the expense of his friends, and sometimes even his lover and future wife, Penny. And while he was the most level-headed of the group, he often showed a mean streak. His witty, biting commentaries are numerous throughout the series' twelve season run, but some deserve special recognition.
"You Might Be From Texas, But I'm From New Jersey"
In the season 5 episode "The Good Guy Fluctuation," Sheldon is hellbent on taking revenge on Howard, Raj and Leonard after they play a Halloween prank on him while at the university. He tries hiding an airbag in Leonard's mailbox, but his plan backfires when Leonard turns it on him.
With Sheldon lying on the floor after the airbag exploded in his face, Leonard triumphantly celebrates, making it clear that his street-smarts can sometimes equal those of Penny.
"A Couple Of Weirdos, Why?"
During the season 5 episode "The Flaming Spittoon Acquisition," otherwise known as the one where Sheldon and Amy become an official couple, the guys take Amy to the comic book store.
Seeing the neuroscientist awakens Stuart's interest, who approaches Leonard to inquire about Sheldon and Amy's relationship. "Are they a couple?" he asks, to which Leonard replies with this quote, making both Raj and Howard laugh. At this time, Amy is still somewhat new to the guys' lives, but her many eccentricities make her a perfect match for Sheldon, something that Leonard knows better than anyone.
"The Same Reason We Don't Do A Lot Of Fun Stuff - You"
After Howard finds out that Bernadette is pregnant, he freaks out because he believes he's not ready to have a child. To help him relax, Leonard, Raj, and Sheldon take him to a tiki bar, where they toast his upcoming fatherhood and talk about what the future holds for them.
Sheldon goes a little crazy with the drinks, rapidly getting drunk. He's having such a good time that, when he asks how come they never went there before and receives this quote as a reply from Leonard, he doesn't throw one of his usual tantrums.
"Homo Habilis Discovering His Opposable Thumbs Say What?"
Way back in the season 1 episode "The Middle-Earth Paradigm," Leonard is invited to Penny's Halloween party, which he attends dressed as a Hobbit. When coming across Kurt, Penny's bully of an ex-boyfriend, who removed Leonard and Sheldon's pants in the pilot episode, he stands up to him by mocking Kurt's lack of intelligence.
It's nice to see Leonard fighting back using his intelligence. While the confrontation doesn't exactly end well for him, he still earns Penny and the audience's respect.
"Sometimes Your Movements Are So Lifelike, I Forget You're Not A Real Boy!"
While arriving at the university one morning, in the fourth episode of season 4, "The Hot Troll Deviation," Leonard tells Sheldon about the drama going on with Howard and Bernadette, and into which Penny was dragged.
Confused, Sheldon asks "Why on Earth are you telling me all this?" to which a genuinely confused Leonard replies with this particular quote. It's both amusing and insulting, and Sheldon actually reacts in accordance, with his face reflecting a mix of offense and slight anger.
"You Call That A Glow Stick? THAT Is A Glow Stick!"
Leonard and Sheldon's friendship goes through a lot of highs and lows on the show, mainly because of Sheldon's difficult attitude and inability to compromise. During the season 5 episode "The Friendship Contraction," after Sheldon keeps Leonard awake all night, the latter decides he's finally had enough of the former's antics and formally breaks the Roommate Agreement.
This provokes an unbalance in Sheldon's life, which he tries to mend by pulling the main circuit breaker switch, causing the power to go out. He then tries to win Leonard's friendship back by luring him with the BBC series Red Dwarf and glow sticks, which he says are recommended by the city of Pasadena.
Leonard rejects his offer, but first mocks Sheldon's tiny glow stick by retrieving his Star Wars lightsaber, calling it an actual glow stick, before returning with Penny to drink wine and pop bubble wrap.
"Yeah, And They Can All Move In With You And Your Mother. The Current Mrs. Wolowitz"
In the episode "The Panty Piñata Polarization," after Penny introduces the guys to America's Next Top Model, Howard is immediately hooked. Looking at the models, Howard calls several of them "the future Mrs. Wolowitz."
Leonard takes the opportunity to mock Howard's delusions while also taking a dig at his living situation. It's a rather successful insult, one made better by Johnny Galecki's perfect delivery.
"We? No, No, No, You Had Your Chance To Be We For Like A Year And A Half Now. Right Now, You Are You And You Are Sc**wed!"
In the season 2 episode "The Cushion Saturation," Penny accidentally shoots a paintball gun directly at Sheldon's spot, staining the cushion. Desperate, she turns to Leonard for help, who tries to wash the paint off, to no result.
Penny then asks, "What are we gonna do?" prompting Leonard to reply that there is no "we" and that she's alone in that mess. Penny actually finds his self-deprecating quip funny, but quickly turns the subject back to the couch, knowing that if Sheldon finds out, she'll be in a world of trouble.
"You Brought Your Husband To Work, You Know The Rules"
In season 2, episode 15, "The Maternal Capacitance," Leonard's mother, Beverly Hofstadter, comes to visit him. Leonard dreads her arrival because she's overly critical of him and her analytical nature prevents her from showing any affection. When Leonard takes her to Caltech, Howard teases him by talking with Beverly about Leonard's more successful brother and sister.
Angered, Leonard tells Beverly about Raj's selective mutism and Howard's living situation, which makes Beverly conclude that Raj and Howard have a pathological fear of women and spend only spend time together to fulfill their need for intimacy. As Howard and Raj look on bemused, Leonard then provided this perfectly timed retort.
"We're Here To See Koothrappali, Not Kill Batman"
Leonard makes this rather funny commentary after seeing Sheldon's Joker-like smile in the season 2 episode, "The Griffin Equivalency." While going to Raj's office to apologize after he, Sheldon, and Howard offended him, Leonard instructs Sheldon to smile to hide his envy.
This makes Sheldon do his aforementioned grin that Howard describes as "terrifying," and prompting Leonard's quip. Howard suggests "less teeth" and the result is deemed "close enough" by Leonard.