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AC Milan 1991-92
First, Arrigo Sacchi created the greatest team in club history. Then, Fabio Capello made them even better.
As star striker Marco van Basten said at the end of AC Milan's undefeated 1991-92 campaign, "perfection doesn't exist in football, but we came pretty close this season."
Their pursuit of perfection had begun in uncertainty, with Sacchi having stood down as AC Milan boss in 1991 to take charge of the national team and been replaced at the helm by former Rossoneri midfielder Capello.
The latter's only previous top-level coaching experience at San Siro had come during short spells as a caretaker and an assistant but the in-house appointment paid off spectacularly for Milan, with Capello claiming the Scudetto in his first season in charge with an undefeated record.
Capello's first Serie A loss didn't come until spring 1993, after a record-breaking run of 58 games unbeaten.
Milan retained their title that summer and then, 12 months later, made it three in a row before destroying Johann Cruyff's Barcelona 'Dream Team' in the Champions League final in Athens to strengthen their case as one of the finest sides in history.
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Kluivert, Van der Sar, Reiziger, the De Boer twins, Davids, Seedorf, Rijkaard, Blind, Kanu, Overmars, Litmanen – in 1994, Louis van Gaal's Ajax boasted the most complementary and exciting mix of homegrown youngsters and wily veterans the game has ever seen.
They were an irresistible force, sweeping all before them in the Eredivisie and the Champions League.
PSV may have had a budding Brazilian superstar in Ronaldo but Ajax had their own teenage sensation in Kluivert, who hit 18 goals as Ajax won the Dutch title by seven points from Roda JC.
They also went unbeaten in Europe, beating title holders AC Milan not once, not twice but thrice on their way to lifting the Champions League, with Kluivert netting the only goal in a 1-0 win over the Rossoneri in the final in Vienna.
Ajax went desperately close to retaining their crown, only to lose on penalties to Juventus in the 1996 tournament decider, and that signalled the beginning of the end; the break-up of an outstanding squad.
It was a shame because as Jorge Valdano had conceded after watching Real Madrid lose 2-0 at home to Ajax in November 1995, “Ajax aren’t just the team of the 90s; they’re approaching football utopia.”
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Preston North End's double winners of 1888-89 were the first English side to go through a season undefeated but it is worth noting that they were playing in a league of just 12 teams.
Thus, Arsenal's perfect campaign in 2003-04 is slightly more impressive. Indeed, whatever one thinks of Arsene Wenger now, his 'Invincibles' will forever remain a Premier League masterpiece, unblemished and unbeatable.
Billionaire businessman Roman Abramovich had arrived in London in 2003 intent on usurping the Gunners as the capital's top club - Gunners vice-chairman David Dein quipped, "he's parked his Russian tanks on our lawn and is firing £50 notes at us" - but Arsenal decided against splashing the cash, with Jens Lehmann their most notable acquisition, at a cost of just £1.5m.
Instead, they focused their attention on retaining the services of France internationals Patrick Vieira and Robert Pires and that policy paid off, with both playing pivotal roles in the Gunners going the whole season undefeated.
They enjoyed some good fortune along the way, with Ruud van Nistelrooy missing a last-minute penalty for Manchester United at Old Trafford - which provoked a most unsavoury reaction from several Arsenal players - but, for the most part, Lehmann was fantastically well protected by Sol Campbell & Co.
Thierry Henry, though, was Arsenal's difference-maker, an attacker as elegant as he was effective. With his pace and poise, he repeatedly inspired them to victory, most notably in a rousing, come-from-behind win over Liverpool at Highbury and he was the embodiment of the team's mix of perspiration and inspiration.
As a rightly proud Wenger declared after watching his side crowned champions at the home of north London rivals Tottenham, "We've been remarkably consistent, haven't lost a game and we have played stylish football. We have entertained people who just love football."
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Athletic Club 1928-29 & Real Madrid 1931-32
Athletic Club were the first side to go through a Liga season undefeated, in 1929-30, although it is worth noting that only 10 teams participated.
Barcelona had won the inaugural title but they finished second, seven points behind the Basques, who won 12 of their 18 matches, scoring 63 goals along the way.
Real Madrid matched Athletic's feat in 1931-32, though, again, there were still only 10 teams in the league.
Athletic actually won more games than anyone else (11) but they suffered four defeats, whereas Madrid went unbeaten, denying the Basques a third successive title with a record of 10 wins and eight draws.
Another interesting fact is that Los Blancos were known simply as 'Madrid' at the time because, during the Second Spanish Republic, all clubs dropped Royal patronage from their names and removed any crowns from their crests.
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However, new coach Ernesto Valverde quickly got to grips with the task at hand, instilling the kind of pragmatism, discipline and defensive organisation that been so conspicuously absent at Camp Nou in recent seasons.
That newfound solidity and doggedness, coupled with the genius of a reinvigorated and more rounded Lionel Messi, has made Barca unbeatable in La Liga.
Indeed, Real Madrid have been left trailing in their wake, with Zinedine Zidane's side effectively eliminated from the title race the day they were routed 3-0 at home by the Blaugrana just before the winter break.
Atletico tried valiantly to keep in touch, and they were eight minutes away from defeating the Catalans on matchday eight until Luis Suarez intervened, but a 1-0 loss at Barca on March 4 effectively ended their game challenge.
The shocking Champions League collapse at Roma has cast doubt on just how good Valverde's side are but while they certainly aren't capable of the football produced during the Pep Guardiola era, they are on the verge of something those teams never accomplished – a flawless Primera Division campaign – and that would ensure them a fully deserved place in the record books.
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Eusebio had already entered his thirties by the time the 1972-73 Primeira Divisao season got underway but the 'The Black Panther' remained one of the deadliest strikers in world football.
He underlined that fact in devastating fashion, scoring a staggering 40 times in just 28 outings to claim the Golden Shoe for a second time, as The Eagles became the first Portuguese side to go through a league campaign undefeated, winning 28 of their 30 matches to make it three titles in a row for English coach Jimmy Hagan.
"It's been a dream," Brendan Rodgers enthused at the end of Celtic's 2016-17 campaign. "I couldn't have written the script for this season any better."
Indeed, less than 12 months after taking over the club he had supported as a boy, the Northern Irishman had guided The Bhoys to a historic, undefeated domestic treble.
They claimed the League Cup in November, with a 3-0 rout of Aberdeen, before wrapping up the Scottish Premiership on April 2, with a record-breaking eight games to spare.
They ultimately finished 30 points clear at the top of the table, after winning 34 of their 38 games to finish with 106 points, having scored just as many goals.
Celtic completed their treble thanks to an injury-time winner from Tom Rogic in the Scottish Cup final clash with Aberdeen.
"To go unbeaten domestically, to get the treble, it's something that we will look back on forever," the Australian enthused.
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When Antonio Conte took charge of Juventus in the summer of 2011, he stood before his new players at their pre-season training camp and made his intentions clear.
"Lads, we’ve finished seventh each of the last two seasons," he stated. "Crazy stuff: absolutely appalling. I’ve not come here for that. It’s time we stopped being crap."
Juve didn't just stop "being crap"; they became unbeatable, the first side to negotiate a 20-team Serie A without losing a single game.
None of Juve's strikers proved particularly prolific - Alessando Matri was the only one to hit double figures (10) in the league - Conte’s side was founded upon a rock-solid defence of Gianluigi Buffon, Leonardo Bonucci, Andrea Barzagli, Giorgio Chiellini and Stephan Lichtsteiner, with the decision to switch to a 3-5-2 proving one of the most significant formation changes in Italian football history.
He also shrewdly built his midfield around the playmaking brilliance of Andrea Pirlo, who made a mockery of AC Milan’s decision to release him in the summer of 2011 by flourishing alongside the dynamic duo of Claudio Marchisio and Arturo Vidal.
In the end, Juve won the title by four points from the Rossoneri and their historic triumph sparked an unprecedented era of Serie A supremacy, with the Old Lady going on to win six Scudetti in a row.
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Remarkably, the first team to go through an entire Serie A season undefeated were provincial outfit Perugia. Unfortunately for the Umbrians, that incredible feat wasn't enough to win them the Scudetto!
Despite their unblemished record, Perugia could only finish second, three points behind AC Milan, who had lost three times along the way.
Perugia's 'problem' was draws: a remarkable 19 of their 30 matches finished level. Indeed, they won just three away games all season, which ultimately proved the difference between finishing as runners-up and claiming what would have been a first Scudetto.
Sadly, they have never again come so close to lifting the title and presently reside in Serie B.
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Andre Villas-Boas was just 32 years of age when he took charge of Porto in July 2010. However, despite his lack of experience, it quickly became apparent that the Dragons had another 'special' coach on their hands.
Villas-Boas led Porto to a treble in his one and only season at the Dragao before following in the footsteps of his mentor, Jose Mourinho, by moving to Chelsea.
Things did not go quite as well for AVB at Stamford Bridge but it was easy to understand why Roman Abramovich paid a world-record €15 million compensation fee to Porto for their coaching prodigy.
Villas-Boas' Porto side had claimed the Primeira Liga with a record of 27 wins and three draws from their 30 games, conceding just 13 times in the process.
However, he ranked Porto's undefeated league campaign as his most significant feat.
"Arsenal’s achievement in 2004 was remarkable and outstanding," he said of another set of 'Invincibles'. "There were games where we lived on the limit and certainly Arsenal did the same. It will be something extremely difficult to do again."