All three hotels are great and close to the parks. The longest walk to any park from any hotel is at most 10 minutes (if that).
So to break it down:
Grand Californian: Most beautiful decor, arts and crafts theme, a GORGEOUS lobby, a nice themed pool with slide, child care, most centrally located and only a few steps (literally) from either DL or DCA - BUT - has the smallest rooms of the three hotels (not that they're small; they're probably similar in size to the rooms at AKL), and the longest hallways is some cases (you'll want to ask for a room close to the elevators). (Many rooms here have nice views of DCA and a few look over DTD)
Disneyland hotel: The true original hotel with lots of neat history, a super fun themed pool (the Neverland Pool from Peter Pan) with a great slide that the kids will love, large rooms (most are large) with fun Disney and Mickey touches, beautiful greenery with waterfalls and ponds all around the property, a nightly light show, and a fun area with toy boats that you can steer around a pond (my DS 6 loved these - okay, DH and I did too

). (All three towers of the DL hotel offer views of the Neverland Pool, and the Sierra tower offers great views of DTD and the nightly (or weekend) fireworks, especially if you're on the 5th floor or higher.)
Paradise Pier: Nice big rooms with a beach decor, some rooms have a great view of DCA, nice pool area with a great slide (no particular theme to the pool area), a great place to stay if you want to save a few points - But it's the furthest from the parks (if you consider 10 minutes far).
Hope that helps you.
(I wanted to add that pool hopping is not allowed at the GC.)