Excerpts from his website:
Among those are outstanding professors of parochial schools, specially Holy Najaf and Qum. Some of them have attained the level of 'ijtiihad'- competence to deduce independent legal judgment enabling them to assume the office of supreme religious authority.
Others reached lofty levels qualifying them for shouldering the responsibilities of teaching and education. Most distinguished among those towering figures is His Eminence Grand Ayatullah al-Sayyid Ali al-Hussani al-Sistani.
He ranks among the brightest, the most qualified and knowledgeable of Imam al-khu'i's former students. In the following account, we try to paint a picture of this meritorious cleric.
This had been certificated by the late Ayatullah Khu'i and also by
`Allamah shaykh Husayn Hilli who both had confirmed his being a Mujtahid
through two separate certification dated 1960, in which the two
Ayatullahs had appreciated his personality and knowledge.
It is worthy to say that up to that date, Ayatullah Khou'i had never certificated any of his students' knowledge or Ijtihad, except for Ayatullah Sistani and Ayatullah shaykh Ali Falsafi (an eminent `alim in the Hawzah of Mashhad. On the other hand, the famous `Allamah shaykh Agha Buzurg Tehrani wrote a letter to Ayatullah Sistani in 1960 in which he eulogizing him for his intellectual talents on biography and hadith. This means that, our master, Ayatullah Sistani, had been granted his high scientific rank when he was only thirty-one years old.
Ayatullah Sistani began teaching the Kharij stage on Fiqh, Usul,
and biography 34 years ago. He held many lectures about the book titled as "makaasib", and many subjects such as purity, prayers, judgment, khums, and some other rules on Fiqh like usury, Taqiyyah (precaution), and the rule known as "ilzam" (obligation).
Sistani also taught the Usul for three complete courses, some of which
are ready for publication, like his research on the scientific roots
(principles), "ta`adul and taraajeeh", some researches on Fiqh, some
chapters about prayers, the rule of Taqiyyah and ilzam. Many eminent
scholars, such as `Allamah shaykh Mahdi Murwaarid, `Allamah Sayyid
Murtadha Al-Mohri, `Allamah Sayyid Habib Husaynan, `Allamah Sayyid
Murtadha Isfahani, `Allamah Sayyid Ahmad Madadi, `Allamah Shaykh Baqir
Irwaani, and many other teachers in the hawzah, have recorded his
researches. During that, Ayatullah Sistani was busy in compiling
important books and some treatises, in addition to what he had written
on Fiqh and Usul.