Do you want full service or just a Marriott product?
Marriott World Center is the closest full service. Warning, they charge to park and it's a haul to parking!
Closer overall are a couple of other options.
In Lake Buena Vista there is a Courtyard. Personally I think it's the CLOSEST overall. It's fairly nice, but it's several years old. I stay there and have never had an issue.
Also in Lake Buena Vista is the Marriott Village. Pretty close and it has a Courtyard, Fairfield Inn and Spring Hill Suites. Any of these would be good choices, all of them are much newer then the Courtyard Lake Buena Vista.