[Frdsdx26]

Which one is read most commonly? Hafs right?

Yes. The most commonly read is Qiraat Hafs from Imam Asim.

Also, why is one more common than the other 6? According to one 'alim in Qur'an, this is due to 3 factors :

1. khilaf in the recitation in qiraat Hafs is much lesser compared to other riwayah
   eg. only 1 imalah ie in surah Hud, only 1 tashil ie in surah Fussilat, only 1 isymam / raum ie in surah Yusof, only 4 places where there are saktah ie in surah Kahfi, Yaasiin, alHaqqah and alMutaffifiin... and several other khilafs.
2. the sanad riwayah is direct and continuous from Imam Hafs to Rasulullah s.a.w. without wasitah (ie intermediary or persons in between) compared to other riwayah such as Qunbul, alBazzi, Khalaf and Khallad
   ie meaning Imam Hafs learnt from Imam Asim who learnt from his teacher Abu Abdulrahman al-Sulami, who learnt it from Ali ibn Abi Tolib, who learnt it from Rasulullah s.a.w.
3. all the bearer of the sanad are very prominent ulama of Qur'an and were very much trusted during their period
   ie Ali ibn Abi Tolib, need no introduction to his well known and indepth knowledge of the Qur'an
   Abu Abdulrahman al-Sulami, a trusted personality who was Hadrat Uthman ibn 'Affan r.a. envoy to the people of Kufah with the mushaf of Qur'an
   When Imam Abu Abdul Rahman passed away, Imam Asim Abu al-Najud became the reference for the people of Kufah

wallaahu a'lam