#4 Sustain their title charge over the years
Manchester United's treble-winning 1998/99 season is considered one of their finest campaigns under Sir Alex Ferguson. However, the Scotsman's greatest-ever accomplishment cannot be narrowed down to a single term. Instead, his ability to sustain a title charge over the years while also challenging for other domestic and international trophies consistently is what makes his Red Devils team the greatest Premier League side in history.
Ferguson created a dynasty at Old Trafford which lasted more than two decades. During this time the Manchester giants won thirty-eight trophies, thirteen of which were Premier League titles. Furthermore, the Scot led them to three consecutive titles twice while also winning a traditional treble in a single season.
Manchester United under Ferguson faced the most challenging opponent - time - and came out on top. Many sides have tried to create dynasties of their own and failed. Both Chelsea and Arsenal have won multiple titles in the past but failed to sustain their trophy haul over a considerable period. Manchester City, on the other hand, are relatively new to the scene.
Now, it is Liverpool who have stepped up to the pedestal to challenge Ferguson's Manchester United. Jurgen Klopp's men have been completely dominant this season but are yet to face the test of time. Should they, like their bitter rivals, sustain their charge over the years, then and only then can they be crowned as the greatest Premier League side ever.