Disclaimer: Only have been to WDW once, and only visited HS and MK and DS (DDT) and a couple resorts that time around.
Hidden Gem: I might get some flack for this one, but... Planet Hollywood.
Was our arrival day visit on our first trip and the best experience. To be honest, couldn't tell you what I ate that day, although it was good. But the service was fantastic; the server and waiter really went out of their way to make our first visit special. My DD accidentally knocked a huge drink all over herself and the table and long story short, they ended up gifting us a HUGE dessert to split and the manager insisted on reimbursing me for the $30 shorts I had to run and buy for DD from the gift shop (even though I repeatedly turned him down, as it was OUR fault, he was persistent! lol) I wasn't planning on coming back on our next trip solely for the fact that I'm trying not to repeat ADRs, but any time we talk about Disney, this is brought up and they want to go back. I hope the new refurb makes it even better!
Must visit: I guess I'll have to go with Crystal Palace.
The buffet was decent and plentiful for both breakfast and dinner, but really, the location can't be beat. An early breakfast ADR can really work in your favor, and if you have a late dinner, you may watch Wishes from the porch.
Would rather forget: Cape May dinner buffet.
Didn't think the seafood was very fresh, and didn't feel there were very many non-seafood options.
Also, most counter service options. "Theme Park Food", yes, but daaaaaaaaaaaaaaang some of it was bad. Going to make it my mission next time to find more palatable QS options.