If you take the first resort bus that you see you can get to any of the parks, however there are some resorts that DON'T have buses to a certain park. There are no MK buses from Contemp, Poly, or Grand flo; they are on the monorail, but be aware the monorail doesn't run super late for EMH like it used to at which point there will be a bus route for it (confusing enough yet?). There are no buses to/from Epcot at the Boardwalk, Swan/Dolphin, Yacht & Beach Club because they are within walking distance of the International Gateway entrance in the World Showcase.
If you're going to Magic Kingdom go for the Poly or Grand Flo bus, if you're leaving MK, go to the Contemp.
If you're going to Epcot, take the bus to the Boardwalk then walk across the bridge to the International Gateway entrance. If you're going to future world, take any bus to a resort then bus there, but bus to Poly then walk to TTC, monorail to Epcot MIGHT be fastest.
If you're going to DHS, bus to Boardwalk then bus to DHS, not sure what other resorts it stops at but it's right across the street so it should be fast even if it's on a loop.
DAK? AKL. Next question...
There is always the option to walk around the end of the lake that DTD is on to Saratoga Springs and take any park bus from there. It's about a half mile from the La Nouba theater to the bus loop there