Biggest problem I have with naming 'hidden gem' restaurants in threads like this is that the restaurant menus and chefs, and sometimes entire concept, change so often...so a restaurant that I suddenly find is an amazing hidden gem with incredible food, 6 months later ranges from mediocre to downright terrible. Then the following year, might be good again.
It's happened to me with a few restaurants that I used to avoid, then heard about a new chef or concept, tried it and found it to be excellent, put it on my list of restaurants to always check out when I go back, only to return and find that everything changed and it was terrible. A decade or two ago when Maya Grill first opened, I thought it was one of the best hidden gems in DW - excellent food, always easy to snag a reservation. Then at some point, it went from steakhouse/Central American spiced foods to American Mexican family food that was like a bland Taco Bell - it was horrible. Then, it got a little better, stayed Mexican but slightly more spices and the occasional Mexican Mexican menu item...not great, but an option to consider. Then it was bad again. Then it went TexMex with a little steakhouse in its current iteration - it's not bad, but not back to the original glory - I'd consider it again in a pinch.
Sebastian's Bistro was the biggest surprise - it was in my opinion terrible, wouldn't consider going there, until just before the pandemic - it ditched the cheap family style buffet-ish food and added some actual table service items, and the chef went bold on the spices and Caribbean flavors...I tried it and fell in love - I knew I had a new restaurant to hit on all my future visits. After the pandemic, I saw they were doing family-style again - but it SOUNDED like it might have kept the spices and such, so I decided to go again - and I hated it. Everything was bland, overcooked, meats so tough I couldn't get the knife through, foods tasted warmed up and cooked hours ago...it was almost inedible and I ate so little of it that I ended up grabbing a quick serve later that night. Off the list it went.
Turf Club has been through some big swings - originally an excellent steakhouse and hidden gem, then it genericized for a while and went to chicken fingers and burgers that were barely a notch above counter-serve fare...then back to a bit more of a proper steakhouse. I occasionally add it back to my list and try, worried each time it will change - as of the last visit last year, it was pretty solid and back to a good underrated spot.
Currently, my list of decent, underrated or little-mentioned spots that I still like to go to, would be Turf Club, Trattoria Al Forno, Three Bridges, Toledo, Sanaa, Ale and Compass, House of Blues, and Jiko. They range from signature to middle price to standard fare - but I can usually get a reservation when other places are booked, and I always come away filled and satisfied with the meal. In some cases, such as Jiko, it has consistently been among my top 1-2 restaurants in Disney, period.
Caveat - any of the above in my list could change tomorrow, and be terrible! I hope not.