5 clubs to go an entire season unbeaten in Europe's top five leagues 

#3 AC Milan (1991-1992) - Serie A

AC Milan dominated their era

Several clubs can genuinely lay claim to being the best of all time, with sides like Real Madrid of the 1950s, Ajax and Bayern Munich of the 1970s, and Santos of the 1960s, all having legitimate shots at the title.

However, two generally accepted sides are the AC Milan team of 1988 to 1992 and the Barcelona team of 2008 to 2012.

There were lots of similarities between both sides, including having the best player of the generation augmented by a host of other world class players, a legendary coach with a revolutionary playing pattern, and titles, lots of them.

While the two sides defined their era and set unprecedented feats, the AC Milan team achieved something that the iconic Barcelona side of Pep Guardiola did not - finishing an entire league campaign undefeated.

Although Ariggo Sacchi helped lay the foundation for the success to come, Fabio Capello helped take it to new heights and under him, the Rossonerri were forged into an almost unbeatable outfit, strolling their way to consecutive titles in what was unarguably the strongest league in the world at the time.

With legendary players like Marco Van Basten, Paolo Maldini, and Franco Baresi, Milan finished the entire 1991-1992 season undefeated, winning 22 and drawing 12 of their 34 league games, with 74 goals scored and 21 conceded.

They went on a run of an amazing 58 matches without defeat (still a Serie A record) and won the next two Scudetti.