It depends what time of day you're going. If you're going while parks are open, take the monorail or boat from the GF to the MK, and then switch to the YC/BC bus. To get back, simply reverse the order.
If you're going for dinner, you need to find out what time the MK closes. If the MK closes early, you'll need to take a bus from BC to Downtown Disney, then transfer to a GF bus.
Another option after the MK closes, assuming you have park hopper's, would be to walk through Epcot and take the monorail from there back to the TTC then switch to the resort monorail. It would most likely still be quicker than going out to DD and back. The buses that run to DD are notoriously slow.
Keep in mind that if you choose the walk through Epcot route, it's about 1 1/4 miles of walking each way, and you need to use a day on your park pass if you have hoppers and won't be entering another park that day.
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