The awesome scene near the beginning of the series when he helps out a student by barging into her parents house and knocking down a wall with a sledgehammer! The audience also got fantastic shirtless fan service in that scene.
There's the moment he saved Murai from the bullies after getting him to show his guts. His method of dealing with the bullies?
Asuza's comes when she calls out the girls that have been making her life hell, hands them shaken-up beers to bring them down a peg, then drinks them under the table just to teach them a lesson in who's more experienced.
Urumi cements her position as a Magnificent Bastard when she sells the virginity of the girls — classmates — who framed 'her' Onizuka by stealing the money for the school trip to some middle-aged perverts for the exact money needed. Evil, but you have to admire her style.
Urumi(in French): For the crime of threatening me, you shall receive my due compensation. The little princess, knowing not the way of the world, will learn in time.
Onizuka also saves a schoolgirl from being held hostage, getting shot up in the process. The whole scenario was a minor distraction for his primary objective of the moment, getting to school to take a 5 hour test. He arrives 4 hours late and aces the test (getting the highest score in the country) while still pumped full of lead.
Kikuchi comments that it's not possible for Onizuka to have gotten a perfect score, since HE HIMSELF couldn't even come close. We never actually find out what his real score was when Kikuchi graded the test afterward, but he had a surprised look on his face. Whatever actually happened is deliberately left to speculation.
He got an 83,85,89,91, and 96 on his last practice test. Considering his countless miracle success and how he does better when push come to shove. Its far more likely he got a perfect score.
The moment when Onizuka beats 99 people in a row at arm wrestling, some two or three at a time, including HEIHACHI MISHIMA. Not least because the final person was Mayuri, Onizuka's latest tormentor, who, when confronted, gave up.
Even before the battle, Onizuka FLICKS a man FROM THE STAGE TO THE CLUB'S WALL. What won't we give to have his two fingers.
In the manga, Onizuka finally snaps and gave Uchiyamada a speech (and a kick in the face) about what he had become as a teacher, who keeps blabbering about reputation and his elevated status as a teacher while Urumi is freezing in a ski dome in a suicide attempt. This left such an impression on Uchiyamada that he decides to rescue Onizuka from his own scheme to get Onizuka fired.
Uchiyamada sacrificing his beloved Cresta to save Onizuka and Miyabi.
Eikichi's confrontation with Teshigawara over his obsession with Azusa. He gets shot at multiple times but he gets RIGHT BACK UP and proceeds to head right to Teshigawara's hiding place (complete with a change of clothes) and beat some sense into him. He follows up by disabling all of Teshigawara's dynamites so he can't blow things up anymore and to climax with LEAPING OFF THE TOP OF THE SCHOOL BUILDING (effectively a good 4-5 stories high at least) to save the guy from commiting suicide by jumping. And he did it all without hesitation. The man is MADE of balls.
Onizuka getting a perfect score in a light gun arcade shooter by dual wielding.
The small and weak Noboru saving the larger Anko from drowning. Despite their past history.
In 14 Shonan he manages to organize a biker gang to helping him search for the police commissioner's kidnapped daughter, then gets the police force themselves to assist, and after she's rescued and the kidnappers busted, the leader tries to run him over with a minivan. Onizuka simply jumps up and kicks through the windshield to smash the guy's face in.
"You're just a shitbag who snatched away someone's precious daughter. THERE'S NO WAY I'D LET YOU ESCAPE!!"
In the live version, Onizuka, upon learning that the school's owners are planning to sell the land, takes the entire class and holes up in their room (complete with video of Tomoko, who has since won a part in a horror movie). This single act makes the other teachers not only realise that he's actually a very good teacher (if hopeless at actual curriculum), but reminds them of why they started teaching in the first place. Then they hole up outside, effectively saving the school. And all this while Onizuka is still recovering from a knife wound. There is a reasonThat Other Wiki says that that episode is so highly rated.
Onizuka beating up the guys who tried to rape Tokiwa, then making the other teachers look like they saved the day.
The final part of the manga where he comes back to life, escapes from his hospital bed, steals a bike, drives it into a burning building to rescue the headmistress who tried to have him run out of school, then motors out of a top-floor window with her on his back. Laughing. Yeah. Rule of Cool.