Quick answer
Вку́сно!
vkúsno — one word, and it will make your host’s evening
The options
Вку́сно!Delicious!
О́чень вку́сно!Really delicious!
Как вку́сно!How tasty!
Прия́тного аппети́та!Enjoy your meal! — said before eating
Я нае́лся. / Я нае́лась.I’m full. — man / woman
Спаси́бо, бы́ло о́чень вку́сно.Thank you, it was delicious.
The second helping
Refusing food once is read as politeness, not as a no. If you really are full, say Спаси́бо, я нае́лся with a smile — and expect to be offered again anyway.
What you hear back
На здоро́вье!You’re welcome! — after a compliment about food
Ещё немно́жко?A little more?
Бери́те, не стесня́йтесь!Help yourself, don’t be shy!
Typical mistakes
Э́то есть вку́сно.
Э́то вку́сно.
No “is” in the present tense.
Вку́сный! — about the meal in general
Вку́сно!
The neuter short form вку́сно works as “it is tasty”; вку́сный needs a masculine noun.
Check yourself
Your host asks how the food is. What do you say?
The neuter short form: вку́сно. На здоро́вье is what the host answers.