Yes, Emma Stone is the biggest fan of the Spice Girls, because anyone who would go to the length to cry over the Spice Girls has to be their biggest fan. Especially in this day and age. The Spice Girls were popular ten years ago, but she cried about them recently. She's their biggest fan.
Emma Stone is the Spice Girls biggest fan. She is claiming to be their biggest fan and broke down into tears when she is asked to talk about it. I think that if anyone can do that about a group that was back in the 90s then you know they are telling the truth.
In order for Emma Stone to be the Spice Girls' biggest fan, there must be no one else who is a bigger/better fan than she is. There are undoubtedly individuals who are obsessive and extreme in their worship of the Spice Girls and Emma Stone does not fit this description. It is fair to say, perhaps, that Emma Stone is a big fan of the Spice Girls, but to claim the title of biggest fan ignores the individuals who take their status as fan to unhealthy levels.
It would be hard to say if Emma Stone or anyone else is the Spice Girl's biggest fan. There are undoubtedly many people out there who consider themselves huge fans of the Spice Girl's and they probably each would think that there is no one else out there who could possibly love them as much as they do.