I don't use WDW buses, so I can't speak for those, though I understand them to be slow.
The fastest way, IMHO, is driving if you have access to a car.
There is a boat that goes from IG at Epcot to the entrance to HS. It's a pleasant ride, except for midsummer afternoons when it can get really hot on the boat (and crowded as heck). It's about a 30 minute ride because it makes several stops along the way.