What dua do I say for my host after a meal?

The main dua for your host

'Abdullah ibn Busr's father invited the Prophet ﷺ for a meal. When the Prophet ﷺ was leaving, he was asked to make dua for the family, and he said: "Allahumma barik lahum fima razaqtahum, waghfir lahum warhamhum" (Sahih Muslim 2042). It asks for three things: blessing in their provision, forgiveness and mercy. That's a generous return for one plate of food.

Other duas from the sunnah

• "Allahumma at'im man at'amani, wasqi man saqani" (O Allah, feed the one who fed me and give drink to the one who gave me drink). The Prophet ﷺ said this when he found his share of milk gone, and a companion then went to milk the goats for him (Sahih Muslim 2055).
• "Aftara 'indakumus-sa'imun, wa akala ta'amakumul-abrar, wa sallat 'alaykumul-mala'ikah" (May fasting people break their fast with you, the righteous eat your food, and the angels pray for you). The Prophet ﷺ said it at the house of Sa'd ibn 'Ubadah (Sunan Abi Dawud 3854). It's especially fitting at an iftar invitation.

Don't forget to thank Allah too

Your host is the means, but Allah is the Provider. After eating, say "Alhamdulillah", or the fuller sunnah dua: "Alhamdulillahil-ladhi at'amani hadha wa razaqanihi min ghayri hawlin minni wa la quwwah" (Praise be to Allah who fed me this and provided it for me without any power or strength on my part) (Jami' at-Tirmidhi 3458). See What dua do I say before eating?

Thanking people is part of faith

The Prophet ﷺ said: "Whoever does not thank people has not thanked Allah" (Sunan Abi Dawud 4811). So after the dua, say a clear "Thank you, that was delicious", compliment the cooking, and offer to help clear up. If your hosts aren't Muslim, such as your parents or colleagues, you can still thank them warmly and make dua for them in your heart, including for guidance. See Can I say grace at Thanksgiving dinner as a Muslim?