とてもおいしかったです。

Totemo oishikatta desu.

That was delicious.

Word for word

“It was very tasty.”

Politeness: Polite

です/ます style. The safe default with strangers, shop staff and colleagues.

When you would say it

After a meal a host or chef made for you.

Worth knowing

Past tense is natural because the meal is over.