That's a tough question because it really depends upon what one orders from the menu, how hungry one is at the time, ect.
But the quick answer, based upon many experiences at WDW resturants is:
TIE between the House of Blues, the California Grill and Mexico's San Angel Inn.
IF you had asked for the WORST dining experience, there would be no tie involved. Easily the worst is the Earl of Sandwich at Disney Downtown.
Enjoy