It's difficult to pick out a number one Ultras group. When you think about it, there are just so many intangibles to be taken into consideration and at the end of the day, a lot of it is simply a matter of opinion.
That said, if you're going to pick one, it's hard to look beyond those of Olympique de Marseille, or OM.
Known for being among the most passionate supporters in the world - if not the most - the displays from the fans are beyond compare, and the atmosphere at the Vélodrome for big matches (especially those in the Champions League or against PSG) is unrivaled anywhere else in Europe.
Setting foot inside the city for any amount of time will almost immediately give you an idea of just how much their team means to the people of the city.