Hmm, this is tough without really knowing your food preferences and price point- here's my two cents for an adult only meal pick if it were me and my DH for each park and some resorts- keeping in mind it's for your anniversary so maybe choosing pricier than normal meals than I would normally choose. We are not fancy eaters when we go since we always have kids in tow.
These are in my preference from first choice to last choice----
Contemporary Resort- California Grille, IMO this would be our #1 choice, make an ADR during the fireworks show and you'll have a great experience. Consistently great food but prices to match. Atmosphere matches the resort but the view makes up for what the restaurant ambiance lacks.
AKL- hit up Jiko- great food and atmosphere but pricey for sure.
Epcot- Le Cellier, we did this restaurant for lunch with our girls, that was a mistake, dinner only would be my future preference. Not terribly uniquely themed but good food and service, definitely more for adults.
MK- Liberty Tree Tavern, fun casual dining, no frills but offers a uniquely themed experience in historic america with typical american food.
DHS- No experience eating at the brown derby so no comment there but there are no other restaurants at DHS worth an anniversary meal IMO.
Like I said we don't usually do signature dining but these (although not all signature) are the ones we've frequented and have had consistently good service.