If CRV opens as it's rumored to, it will be a monorail DVC closest to MK.
Even still, with the general layout of WDW it doesn't look possible to have one DVC close to ALL 4 parks. BWV, and BCV are the closest DVC resorts to Disney Studios (formerly MGM), and Epcot. The Boardwalk area is actually in between them. WLV is (as above post) a boat ride from MK, therefore you couldl take the boat to MK, and jump on the monorail to EPCOT.
As everyone had mentioned, BCV and BWV are the only ones that are walking distance to any of the themeparks --Epcot and Hollywood Studios (MGM - I think I'll always consider it that). Although AKV is very close to AK, sadly there is no walking path. If there was one, AKV/AKL would be considered a more convenient location (even if it's just to AK).
And this is one of the main reasons why CRV (if it does materialize) would also be a very popular DVC resort since it would be on the monorail.