In the summer of 1999, an American mafia drama called The Sopranos aired for the first time. It's only now, 20 years on, that we're able to fully assess the impact it has had on our cultural landscape, this TV show that changed TV for the better.

Obviously, it paved the way for Walter White, Don Draper, Stringer Bell and all the other flawed examples of masculinity we came to expect as standard from our heavy hitting TV dramas. Before Tony Soprano, leading male characters on the small screen were fantasies: brave policemen, perfect Fathers, men whose actions and morals fitted neatly into cosy story arcs that repeated, week in week out.

The Sopranos changed that forever. It made TV grow up. David Chase and HBO proved audiences in their living room were far smarter than they'd ever been given credit for, and ever since, studios have upped their game, writers have stopped seeing TV as a stepping-stone to cinema, and Hollywood's most ambitious actors have fallen over themselves to find a small screen role as profound, challenging and as loved as Tony Soprano.

But let's not forget one thing. Aside from transforming the TV landscape, what James Gandolfini, David Chase and everyone else involved in the show left behind was six seasons of drama that gets better every time you watch it, from the dated but still brilliant first series to the daring, artistic flourishes of the last, The Sopranos remains the most absorbing, funny, shake-your-head brilliant show ever made.

Here, to mark its anniversary, we round up the 15 moments that made The Sopranos what it was. There are spoilers, of course, but if you still haven't seen it after all this time, we have to ask: uff marone, what the hell are you waiting for?!

15. Pussy Gets Whacked (Season Two)

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The first major death in The Sopranos came at the end of season 2, when Sal "Big Pussy" Bonpensiero was ousted as a rat and murdered aboard Tony's boat. It's a queasy scene - and not just because of Tony's food poisoning - as the guys come to terms with having to kill their best friend. Pussy's death reverberated throughout the rest of show, and made the important point that on Chase's watch, no character was safe.

14. Tony's Still The Boss (Season Six)

In a bid to reassert his authority after being left weak from a near-fatal shooting, Tony provokes his new bodyguard – a body builder with a hot temper called Perry Annunziata – into a fist fight in front of the crew. Tony comes out of top, breaking the younger man's nose before retreating to the privacy of the bathroom, where he duly vomits blood. The smile on Tony's face as he looks in the mirror is priceless: in a world where violence rules and appearance is everything, the old lion had proved he still has what it takes.

13. Ralph Kills Tracee (Season Three)

One of the most controversial and hardest to watch scenes in the entire show came when Ralphie Cifaretto beat a young woman he'd been dating to death at the back of the Bing. It is, however, a heartening and revealing moment when an appalled Tony breaks with mafia protocol and reacts by giving Ralphie a beating of his own. A crucial scene in calibrating Tony's morality, it also marks the end of Tracee, a minor character viewers took into their hearts during her few episodes.

12. The Intervention (Season Four)

The Sopranos was rarely on better comedic form than when the 'old school' world of the cosa nostra clashed with the touchy-feely new world of New York liberalism. The group intervention over Chrissie's heroin problem – one of the rare scenes to include eight regular cast members at the same time – is a perfect example, with Paulie and Silvo's idea of 'compassionate sharing' a particular highlight.

11. Tony Kills A Rat (Season One)

'College' was a hugely significant episode from the show's debut season, mainly for this scene. While taking his daughter Meadow on a tour of potential schools, Tony stumbles upon a former rat and decides to take revenge (literally) into his own hands by strangling him to death. It was the first time audiences glimpsed the depths to which Chase was prepared to allow his anti-hero to plunge. HBO was nervous the scene would prove too much and ratings would suffer. They didn't. Note also Tony's glance upwards at the end of the scene at some flying ducks, a symbol of his family – this was the first episode where his 'two worlds' came dangerously close to colliding.

10. Janice Kills Richie (Season Two)

There are few more formidable characters in the entire of The Sopranos than the newly released from jail and resolutely 'old school' Richie Aprile. A constant menace, he nevertheless meets his match when he assaults his fiancé (and Tony's sister) Janice, who promptly responds by putting a bullet in his chest. One problem solved for Tony, and a reminder that it's not just the men in the Soprano family with the killer instinct.

9. Remember The Little Moments (Season One)

"I'd like to propose a toast. To my family. Some day soon, you're going to have families of your own, and if you're lucky, you'll remember the little moments, like this… that were good."

Tony Soprano is not much of philosopher, but he has his moments. This scene in which he, Carmela, Meadow and Anthony Junior take refuge in Vesuvio's after being caught in a storm at the end of the show's first season is one of them. His words are echoed later, first by Meadow in the wake of Jackie Junior's death and then by AJ, during the show's finale. It's also a good summation of the epiphany Tony has after coming out his coma in season 6. In other words, David Chase knew what he was doing all along and planted the seeds from the very start, the crafty so-and-so.

8. Adriana Gets Whacked (Season Five)

Adriana La Cerva was only ever meant to be a minor Sopranos character, but so impressive was Drea de Matteo as Christopher's spirited but naïve girlfriend, they gave her the biggest storyline of season 5 when she was turned into a reluctant FBI informant. Fans had seen her death coming the second she was 'flipped', of course, but it didn't make watching her scurry into the woods on all fours – with a menacing Silvo taking aim behind her – any less harrowing.

7. Chris And Paulie Get Lost (Season Three)

Picking a highlight from 'Pine Barrens' – the Steve Buscemi-directed episode regularly voted Soprano fans' all time favourite – is futile. The entire 50 minutes is hilarious, from Paulie's wholly unnecessarily provocation of the Russian (which ultimately leads them to getting lost in the New Jersey woods) to the pair's delirious arguments (above). Actually, we can pick a highlight: watching Paulie's silver wings gradually deteriorate at the ordeal goes on. Get that man some Brylcream.

6. AJ's Suicide Attempt (Season Six)

Anthony Junior's journey from goofy kid to deeply troubled adolescent is complete when he makes a typically botched attempt to kill himself in the Soprano family pool. Arriving home in the nick of time, Tony's running jump – in a full suit – to save his son is heartbreaking. But the scene (and much of the season) belongs to Robert Iler, who proves the producers were right to take a chance on him as a kid all those years ago.

5. Adriana's Confession (Season Five)

James Gandolfini and Edie Falco took most of the plaudits for the acting in The Sopranos, but close behind them was Michael Imperioli whose own Emmy-winning highlight was this scene, when his girlfriend Adrianna reveals she has been working with the FBI. Bewilderment, fear and terrible violence pour out of him inside three virtuoso – and incredibly difficult to watch – minutes.

4. Tony Fights Off 'Car Jackers' (Season One)

The Sopranos often made sublime use of music. In 'Isabella', a morose Tony buys a cartoon of orange juice (a nod to the Godfather) and walks to his car to the melodramatic sound of 'Tiny Tears' by Tindersticks. Seconds later an attempted hit on his life by two armed thugs begins, and the music abruptly stops. In a brilliantly orchestrated scene, Tony snaps out of his malaise and fights them off, before laughing at his brush with death. As he says later to Dr Melfi: "Talk about a jolt to the system. Try gettin' shot at … Well, i'll tell you somethin'. I didn't wanna die. Every fuckin' particle of my bein' was fightin' to live." An epiphany, Sopranos-style.

3. Melfi Almost Tells Tony (Season Three)

Doctor Melfi's psychiatry office is the setting for countless revelatory moments, primarily Tony's. But at the conclusion of episode 'Employee of the Month', the roles are reversed to devastating effect. After Mefli is the victim of a rape – by a man subsequently let free due to a police blunder – the doctor and patient meet for their usual session. Overcome with trauma and for once, not wearing her glasses, Melfi becomes overwhelmed and begins to cry, prompting a confused Tony to ask if he can do anything to help. With one word, Melfi knows she can have her attacker torn to pieces. The moment lingers unbearable. The audience wills her to say 'yes'. She says no – and the screen cuts abruptly to black.

In this scene, Chase is preserving two crucial elements of the show. First, that Melfi is its moral centre. She knows unleashing Tony will make her an accessory to his crimes, and unlike almost any other character in the show, prizes her integrity above her impulses and desires. Second, that the show will never gives us what we 'want', i.e. to see Melfi avenged. This principle is upheld, as we were to discover, throughout The Sopranos, right up to the show's final scene.

2. The Final Scene (Season Six)

Let's clear one thing up from the start. If you didn't like the ending, you didn't like The Sopranos. Not really. Not enough. You might have thought the show was cool, and enjoyed the wise guy stuff, but if you watched the now famous black out as Tony and his family sat in a New Jersey café and thought, 'I've been robbed', then you really weren't paying attention.

For Chase to have ended the show in a hail of bullets with Tony's corpse on the floor – or for that matter, with Tony living some form of happy ever after – would have been a betrayal of everything The Sopranos stood for, a return to the very TV conventions it so boldly subverted in the first place. So no, you don't get to find out if Tony is dead, or merely living on in a state of heightened paranoia. There is no easy resolution, no redemption, no lessons learned, because life isn't like that either.

The final scene of The Sopranos is, however, a gloriously cryptic piece of art, laden with possible symbols, clues and red herrings that have had people pouring over it for the past decade, reading into them more or less whatever they wish. It's a fitting crown to a show that transformed TV into the dominant American art form of the century, and they'll be studying and arguing over it in classrooms for many years to come.

1. Tony And Carmela Fight (Season Four)

Of Tony's two families, it's the wise guys who offer viewers the most fun: the gags, the gambling, the guns. But for anyone properly invested in the show over its 86 episodes, the real emotional impact is in the scenes at home, with his other family.

At the heart of this were James Gandolfini and Edie Falco, two actors playing a married couple who were at once deeply in love and disintegrating over the usual things: money, secrets and – above all – infidelity. The fourth season of the show is sometimes thought of as the least eventful, but that's a misconception. The unraveling of the Sopranos marriage, the end of their decades of uneasy truce, is in many ways the most dramatic 'death' in the show.

In 'Whitecaps', the episode that won Gandolfini and Falco an Emmy a piece, Tony and Carmela go at it in a way only long-married couples can, hurling every spiteful thought and horrible home truth they can muster at each other like so many pots and pans. It's two of TV's greatest performers at the very peak of their powers, and it demonstrates precisely why The Sopranos transcended the thrills of the gangster genre to become something timeless and touching. You watch it, marveling at the script and the performances, all the while unable to believe for a second that these aren't real people, with real lives.