I'm going to say hands down, Polynesian.
While it is one of the most expensive deluxe resorts (just below GF in pricing) if you go later in September or after Christmas you can beat the holiday crowds AND rates. For timing my family has found that mid-November or the first week of January are perfect if you want to experience the magic of WDW at Christmas. You're not dealing with the holiday rates and the parks are less crowded. November allows you to experience MVMCP, but right after New Years is quieter in the parks and the decorations are still up. If you really want the Christmas ambiance and
extremely comfortable temperatures (highs in the low to mid 70's, lows between 40 and 50) go for the first week in January. Note: in 2008 the WDW marathon is January 6 and 7, so there are slightly more crowds from that and longer bus trips due to road closures. We went during the marathon in 2006 and didn't find either of these to be significant issues, especially since the parks were not very crowded to begin with.
However, if you do go during this time, make your reservations as far in advance as possible (ie August) because rooms do fill up quickly for the event.
Now for the wonders of the Poly! We've been to WDW about 10 times now, and every time we stay at the Poly. This resort has the highest rate of return guests of any resort on property. The rooms have all recently been refurnished and are beautiful. The CM's are excessively helpful and friendly and everyone greets each other with a cheerful "Aloha!" There is a very popular themed pool and a quiet pool (conveniently open 24 hours, and from tons of experience I can tell you that it's wonderful to relax in an empty heated pool after a long day at the parks.) along with the lagoon. Unfortunately swimming is no longer allowed in the lagoon, but there's access to water recreation and the beach itself is open. The scenery is magnificent, the Never Land Club and new Mona Mickey's Arcade are available for children, the restaraunts are fantastic and transportation is a breeze. The resort monorail runs directly to MK and the TTC is at most a five minute walk. The Poly has the most authentic "resort" feel, and you often think you're actually in the Pacific instead of FL. I would say this is the most family friendly of any of the MK area resorts, and there must be a reason they have the highest guest return rate! :animwink:
If you do stay here, I suggest trying to get a garden view room in the Niue or Samoa longhouse. While you can't choose your longhouse during the reservation process, the front desk staff will do everything they can to get you into your top choice during check in. Both longhouses are located in between the two pools and are right next to the Grand Ceremonial House. For convenience, these two can't be beat (30 second walk to GCH, 30 seconds to lagoon/either pool, 2 or 3 minutes to TTC, and right near the boat dock if you want to take the water launch to the MK - which I highly recommend).
Can you tell I'm a die-hard Poly fan?