The quickest way from MGM to AK is to take a cab.
The second quickest way is to have a rental car available to you
The third quickest way is the bus. The bus may be just as fast as the cab or rental car, if you get lucky and it is at the stop when you arrive.
The quickest way from MGM to Epcot (International Gateway) is to wear a pair of running shoes and run over.
The next quickest is a cab.
After that a rental car.
Then the boat or the bus. It takes about thirty or fourty minutes to ride the boat between the parks, stopping at the resorts. Plus the wait if the boat isn't there.