We have stayed at almost all of the Marriott properties within 10 miles of WDW as we've done probably 15 trips to Disney in the last 5-6 years. My favorite is the Courtyard on Vineland in Lake Buena Vista at the Marriott Village, followed by the Fairfield and Springhill Suites at the same address. The Courtyard is clean and relatively new and they have rooms with two queens (whereas most others are two doubles). The fairfield and SHS have complimentary breakfast. We like the courtyard because it is well built and maintained with a nice indoor/outdoor pool and it is close to the outlets and Downtown Disney.

Just don't confuse this Courtyard with the nearby LBV Courtyard at Vista Centre which is old and terrible.

Marriott World Center is also nice but expensive (including resort fees and parking) and involves a lot of walking, but a new pool renovation.