I'd say Man Utd but we all know that many of their supporters are "fashion supporters" so their popularity might dwindle if they're empty handed this season again Real definitely comes after that...
True but ManU probably have more supporters in Scandinavia than in Britain... If you mean the whole world there are more ManU fans in places like the Far east than those that speak English.
In the Middle East (Arab Countries) people tend to like the Serie A more, and are always behind the big 3 - Juve, Inter, and Milan .. but in the Far East everyone follows the EPL: TV Coverage, Magazines, Newspapers, TV shows, Football kits, and even official shops (recently) ...