What is the best supplication or verse mentioned by the Messenger, may Allah bless him and grant him peace?

The best verse in the Quran is Ayat al-Kursi (the Throne Verse), as mentioned in the Prophet's (peace be upon him) hadith to Ubayy ibn Ka'b. When the Prophet asked him about the greatest verse in the Book of Allah, and Abu al-Mundhir (Ubayy's kunya) replied that it was Ayat al-Kursi, the Prophet affirmed it. As for the best supplication (dua), the answer to that has been provided previously.