As in the situation when you are expecting your food to be average, but it is really good. "Yummy" probably doesn't cover it.
On the flip side, what adjective would describe food that is unexpectedly bad?
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I think outstanding conveys the surprising/unexpected sense of delight.
For a bad experience, I would definitely call it disappointing, which Merriam-Webster defines as
Edit: An alternative phrase would be to say that it "exceeded my expectations", but if you are trying to compliment the chef, then this might imply that you did not have very high expectations.