My vote is the Poly, especially if you're going to MK and Epcot the most. To MK take the monorail 2 stops or take a boat. To Epcot take a 5 minute walk to TTC and hop on the Epcot monorail (if you take the resort monorail from Poly you have t5o take the entire loop: GF, MK, CR, TTC, before transfering to the Epcot monorail). For MGM and AK it's buses. I know they supposedly shere the buses with one or two other resorts (the arrangement changes often) DW and I were there over Xmas last year and did not have any problems.
CR may be second best b/c you walk to MK (and take monorail 1 stop back). For Epcot you take resort monorail 1 stop to TTC and transfer to Epcot monorail (it's too far to walk from CR to TTC).
As others have said, AKL has direct buses to each park, but it only has buses. Plus, AKL is a little isolated from most of the parks and other resorts. It is the farthest deluxe from MK, and therefore the longest bus ride (but the shortest ride to AK

).
The Epcot resorts (BWI, BC/YC, S/D) have boats to MGM and Epcot, or you can walk. Depending on which resort and which park the walk could be tiring on a hot day. The boats are comfortable, but don't get you there very quickly because they stop at all the resort. Howver, the Epcot resorts seem to be in the middle of everything and very centrally located.
In all honesty, no resort has the "best" transportation for every situation. Whatever resort you pick will have the "best" transportation to 1 or 2 parks, and another resort will have better transportation to other parks. But none of them are bad. You get to the park in comfort and with ease no matter where you stay.