Disney transportation will not take you resort to resort so you will have to go to a park first then take a bus, boat, train to the resort you want to visit. You don't have to go into the park. What I normally do:
Take the bus to MK and then hop on the resort monorail, that will take you to CR, Poly and GF then back to MK. If you still have time, hop on the boat to WL, then back to MK, bus back to CBR.
For Epcot resorts:
Bus to HS, either walk or take the boat to YC/BC, walk around the lake to BW the either boat or walk back to HS and bus back to CBR.
Bus to AK, hop on a bus to AKL then back to AK and back to CBR.
If you want to tour the moderate or value resorts, just about any park will get you there, MK is the furthest away from those resorts so don't start there.
I would not rent a car just to tour resorts, the parking at the deluxe resorts is few and far between even for people staying there.