If you're mobile, the fastest way is to walk. It's about 20 minutes. The path is along the water and you get to it by walking down toward the bus stops - just go straight when you leave the park, head right when you hit the water and keep bearing left around the bend and you'll see the signs. The path goes under the road - there's an overpass over the water and the path is along the water.
You can take the boat - takes about 30-35 minutes with stops at Swan/Dolphin, Yacht/Beach and Boardwalk between. If you head left as you leave the park you'll see the dock. At Epcot you'll enter at the International Gateway entrance.
If you want to split the difference, you can take the boat, get off at the Swan/Dolphin, cross over to the path on the Boardwalk side and walk the rest of the way.
The bus is probably the slowest way, but if you're going to be doing things at the front of Epcot you won't have to walk through from the IG entrance as the bus drops you at the front of Epcot.