The readers of the Qurʾān
The Qurʾān was handed down by recitation, teacher to student, in unbroken chains. Ibn Mujāhid (d. 324 AH) fixed seven readings whose chains and Arabic were beyond doubt; three more were later counted with them, making ten. Each reader is known through two transmitters (rāwīs): most of the Muslim world today reads Ḥafṣ from ʿĀṣim, North and West Africa read Warsh and Qālūn from Nāfiʿ.
The great narrators
The Companions through whom most hadith reached us, with the number of narrations the scholars count for each (the counts are Baqī ibn Makhlad's, as given by Ibn Ḥazm and al-Dhahabī; they include repetitions).
The Successors
The Tābiʿūn met the Companions and carried their knowledge to the next generation: the seven jurists of Medina, the students of Ibn ʿAbbās in Mecca, of Ibn Masʿūd in Kufa, of Anas and Abū Mūsā in Basra.
The four imams
Abū Ḥanīfa, Mālik, al-Shāfiʿī and Aḥmad ibn Ḥanbal, whose schools of law the Sunni world follows to this day. Abū Ḥanīfa and Mālik learned from the Successors; al-Shāfiʿī studied with Mālik, and Aḥmad with al-Shāfiʿī.
The masters of hadith
Those who gathered, sifted and recorded the sayings of the Prophet ﷺ: from Sufyān al-Thawrī and Shuʿba to al-Bukhārī, Muslim and the authors of the Sunan.