My favorite restaurant in ALL of WDW is Fresh at the Dolphin, and almost no one knows about it. I guess that qualifies it as "underrated".
The lunch buffet has a mediterranean theme, and it's not the "usual" buffet food. They don't offer the gyro sandwiches every day, but they're almost heavenly. Whatever they have on a particular day, it's consistently excellent. (Try small helpings of everything - you'll want to try 'em all.)
I wish they served dinner, but it's B & L only.