We own at and always stay at Horizons, so I am biased, but the general consesus appears to be as follows:
The Palms resorts (Imperial, Royal, & Sabal) are the closest to Disney (1 mi to the gate) and allow access to the facilities at the Marriott World Center. These are all older, but nice and the hotel is supposed to have very nice pools, etc. The pools at the resort are very basic and it is a long walk/short drive to the hotel which is hard with really young kids.
Cyprus Harbor is very close to Sea World. It is a mature resort with nice landscaping. It also seems to have a lot of activities, but geared more towards middle school/teens.
Grande Vista is also near Sea World. It is the newest resort and has the Faldo Golf Institute if you like golf. It is supposed to be very nice and is a "full" Marriott with hot tub in the room, etc. as well as a game room and activities for kids.
Horizons is across from Cyprus Harbor and is geared towards families with young children. The rooms are nice, but do not have a hot tub in the room like the other Marriotts. There is a pirate themed ship pool, waterworks and a mini golf course. They also have "pirates" who do activities with the kids every day. What I like is that the resort is small, no more than a 5 minute walk from you room to the pool and there is a bar & grill right there. Everything is easy to get to and geared towards kids. My DDs are 4 and 2 and have been coming since they were 1 years old each and have always had a great time. They are always asking when they can go back and see Captain Horizons.
I don't think you can really go wrong with any of the resorts. Other people can chime in with more specifics about the other resorts.
David