I agree with others about the fastest way being driving or catching a cab directly to GF. But if you want to rely on Disney transportation (knowing it'll take longer), I'd take a bus from BC to MK, and then catch a boat from MK to GF. I say the boat instead of the monorail because the GF/Poly boat will go directly to GF first, while the monorail would have stops at CR, the TTC, and Poly before getting to GF.