The cheapest is Mears, if you are traveling single or as a couple. However, IMHO, it isn't the best way to go. We have had good experiences with Mears (riding in their vans, being the first to be dropped off, short waits) and we have had bad experiences (being one of 50+ people in a bus, having to be picked up 3 or more hours before a flight, nearly missing flights because of numerous stops at other resorts). But if you aren't in a hurry, Mears is the way to go.
We have also used several of the towncar services, mostly good experiences and only one bad experience. They do come at the cost of around $80 RT plus a tip.
And recently we discovered rental cars which is now our preferred method of ground transportation. A weekly rate isn't much more than the towncar in many cases. We now prefer driving to the parks than using Disney transportation. We usually stay in a moderate or value resort.