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

#4 Arsenal (2003-2004) - Premier League

Arsenal did not taste defeat en route their 2003-2004 league title

Arsene Wenger is known as a revolutionary who helped introduce far-reaching techniques that forever altered the Premier League, such as monitoring players' diets and training regimes, advanced scouting techniques, training methods, and focusing on the scientific aspects of the game rather than the physical.

However, his most defining legacy to the English game was when he guided his Arsenal side to an undefeated league campaign in the 2003-2004 season.

Led by a prime Thierry Henry (who finished as the Golden Boot winner), and supported by stars like Robert Pires, Freddie Ljungberg, Nwankwo Kanu, and the energetic skipper Patrick Vieira, Arsenal finished the 2003-2004 campaign as champions, without any defeat in the process.

The Gunners won 26 and drew 12 of their 38 league games, finishing 11 points above second-placed Chelsea.

Thus, they became only the second side in English top-flight history to end a season undefeated (after Preston North End over a century ago) and also hold the record for most undefeated matches on the trot (49) and are affectionately referred to as the Invincibles.