けっこう

けっこう is an adverb meaning "quite", "pretty": the degree is higher than you expected. It goes before an adjective, a verb or another adverb, and is often written in kanji as けっこう結構.

けっこう or とても

とても says the degree is high. けっこう says it is higher than you thought it would be, so けっこうおいしい means the food turned out better than expected.

For the same reason, けっこう in a compliment can sound as if you expected less. Praising someone's work or their Japanese, say とても.

けっこうです

On its own, けっこうです says that something is enough, or acceptable. Offered more food or a service, it means "no, thank you"; asked about a time or a plan, it means "that's fine".