Fastest option early in the day is definitely walking to the Contemporary and taking the MGM bus from there.
The second fastest is taking the MK direct bus to the Boardwalk resort, then walking from there to MGM on the path along the lake. This option may actually be faster later in the day, when more people are leaving parks than heading to them.
Either way, you'll get there faster if you spend the time when you are not on a vehicle walking, rather than waiting to transfer from one vehicle to the next.
I have always found that there tends to be a long wait between the park-to-park buses; I once waited a full hour at Epcot for a direct bus to DAK.
[Edited to correct name of resort: I meant to say the CR, not the Poly.]