Resorts in the Value and Moderate category have separate buses that run to each park, water park and DTD about every 15-20 minutes.
Deluxe resorts in the Magic Kingdom area (Grand Floridian, Polynesian and Contemporary) have a Monorail to take them to Magic Kingdom, and a monorail stop to switch to the Epcot Monorail. These resorts also have boat service to Magic Kingdom and bus to DHS, AK, water parks and DTD.
Deluxe resorts in the Epcot area have a boat to Epcot and DHS, and bus to other parks and DTD.
Wilderness Lodge and Ft. Wilderness have boat service to MK, also bus to MK and all other parks, and DTD.
Animal Kingdom Lodge (Deluxe) is very quick to Animal Kingdom, but it's by bus. There is also a bus to all other destinations.
So you have your choice of resort price level, and some choice in mode of transportation. All resorts have free transport of some kind, and free parking at the resorts and parks. If you stay at MK Deluxe, you have quick access to MK. If you stay at Epcot Deluxe, you have quick access to Epcot/DHS. WDW is so big (in square miles), that there's no such thing as quick access to each of the 4 major parks. We swear by the bus service. It's first class, clean, friendly and very well managed. You won't need a rental car at all.