I would have to go with the Poly. To get to the Magic Kingdom, you can take either the resort monorail (Poly -> GF -> MK) or take the launch from the Poly dock to the Magic Kingdom. To get to Epcot, you could walk to the TTC (about 5-10 minutes depending on which longhouse you're in) and then take the Epcot monorail, or take the resort monorail (Poly -> GF -> MK -> CR -> TTC) and then get the Epcot monorail at the TTC.
Contemporary would be right up there too. You can walk from the Contemporary to the MK (is there a launch also? I don't recall?). Taking the Resort monorail (CR -> TTC -> Poly -> GF -> MK) would most likely take much longer than walking. To go to Epcot, just take the resort monorail from the CR to the TTC and then switch to the Epcot monorail.
Bus service from either resort to MGM and AK is pretty good too, I'm not sure that one is much better than the other.