Overall, here's my pick and why:
1. Yacht Club - best pool(s), good restaurants, proximity to Epcot/MGM, good restaurants on-site and nearby. Great if you like to walk and even if you don't (boats). I love room decor and balconies. Upscale feel but still laid back. Less expensive than other two.
2 (tie). Grand Floridian - on Monorail, afternoon tea, on-site massage, beautiful resort with beach/pools. Very expensive, though and from family I know who has stayed there -- not that impressed with rooms. Obviously, it has a victorian feel. If you do stay, spend the money and get room in main building, not garden wings. Only 5-star restaurant on property. We dine and spa there but prefer to stay elsewhere.
2 (tie). Polynesian - great pool, on monorail, good restaurants, luau, romantic feel. Downside - I don't like room decor that much and you have to walk outside to get from lobby to rooms (vice-versa). I'd pay a little more to get a view of castle/beaches and closer to lobby.
GF and Poly are a tie -- depends on the 'feel' of resort that you want, because they couldn't be more opposite but I like both equally. If you REALLY want to stay on the monorail or close to MK, obviously YC is not for you on this trip.
If you're staying 10 nights or more, I suggest splitting b/w a monorail resort and YC.