MK: There really aren't that many table service options and getting walk-ups seems to be hit-or-miss. I would probably try for LTT or the Plaza Inn. If neither of those work, you might have to consider leaving MK and heading to one of the resorts. For the less busy Signatures, I like Artist Point (great food and outstanding service). For a non-Signature, I would probably look to the Grand Floridian Cafe (I hesitate to "recommend" GFC because while the food was good, but the service was very slow).
Epcot: This has the most options and you can usually get a walk-up at Marrakesh, Nine Dragons, and one of the Japanese places (either Teppan Edo or Tokyo Dining).
DHS: Again, there aren't a lot of table service options. I think your best bet would be to head over to the Epcot resort area. Big River Grille doesn't take ADRs, so they're always available for walk-up. Yachtsman Steakhouse (good, but nothing exceptional) might be an option, as is Captain's Grille (okay food, noisy).
AK: Tusker House seems to always take walk-ups, even when the wait is a hour or more. I like Yak & Yeti or RFC and if you have the Landry Select Club card you don't need an ADR.
DTD: Most all of the DTD venues take walk-ups.