The Hilton and the Grosvenor are closest to Downtown Disney. But the DTB buses are only going to get you to Disney-owned resorts; there are no buses between DTD and any of the theme parks.
The Hotel Plaza Boulevard properties have their OWN bus system that's MUCH more efficient in getting you to the various parks than trying to cross a very wide, very busy street, then boarding a Disney bus, then going to some resorr, then switching to yet another bus to get to a park.