Ayatul Kursi (the Verse of the Throne) is found in Surah Al-Baqarah. It is specifically verse 255 of the second chapter of the Qur'an. Ayatul Kursi is one of the most well-known and frequently recited verses in the Qur'an, and it is considered the greatest verse in the entire Qur'an according to an authentic hadith narrated by Ubayy ibn Ka'b.
The verse describes Allah's absolute sovereignty, power, and knowledge. It begins with "Allahu la ilaha illa Huwa, Al-Hayyul-Qayyum" (Allah - there is no deity except Him, the Ever-Living, the Self-Sustaining). It affirms that Allah's Kursi (Throne/Footstool) extends over the heavens and the earth, and that preserving them does not tire Him. The verse encapsulates core aspects of Tawhid (the oneness of Allah).
Surah Al-Baqarah is the longest surah in the Qur'an with 286 verses. The other surahs listed - Al-Nisa (Chapter 4), Al-Imran (Chapter 3), and Al-Tawbah (Chapter 9) - do not contain Ayatul Kursi, although they each contain important verses on other aspects of Islamic law and belief.