The best dhikr is lā ilāha illa Allah (there is no god but Allah), and the best supplication is Alhamdulillah (praise be to Allah)

The Prophet (may Allah's peace and blessings be upon him) informs us that the best dhikr (remembrance of Allah) is Lā ilāha illa Allah (there is no god but Allah), which means that no one is worthy of worship except Allah; and that the best supplication is Alhamdulillah (praise be to Allah), and it means acknowledgment that Allah Almighty is the Bestower of favors and the One worthy of possessing the attributes of perfection and excellence.