First of all, idk what game are you referring to, but if it is FOOTBALL there isnt a single most important club in the world, clubs live by competing against other clubs, winning silverwear against rivals etc... Der Klassiker, Der Ruhr Derby, El Clasico, El Madrileno, Derby de Cataluna, Le Classique, La Madonnina, Derby d`Italia, Derby Del Capital, North West Derby, The Dockers, North London Derby, Derby de Casablanca... just testifies and points out to how important rivaleries are in the Footballing world, so singling out a club and pretending the footballing world would collapse without it is an open and shut case, Look at Hamburger, Red Star, Stewa Bucharest, Parma, Nott Forest, they once were giants but after their downfall Football continued to exist.
I have a soft sport for FC Barcelona, for me, and me alone as a Barca fan, Barca is the most important club on the planet, and has beem for me since 1996, the team of the Ronaldo, Stoichkov, Figo, Giovanni, Laurent Blanc, Vitor Baia, Luis Enrique, and that for me as an individual, but football will exist even without Barca and Real and Bayern, and Man Utd, and Liverpool and Milan and Juve. The teams I have mentionned are the biggest and greatest teams of all times, please dont come calling PSG, Arsenal, Man City,