Orlando International Airport is by far the most common, due to its closer proximity and abundance of flight options. Orlando Sanford Airport (SFB) is smaller and a bit further from the park entrance.
Sanford Airport (SFB) is about 50 miles or a 1-hour drive to the entrance to Walt Disney World.
Disney no longer offers any kind of free transportation to Disney World. There is free transportation to the theme parks once you get on property.