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as salam alaikum

there are different types of tafsir. One is the tafsir based on reports, mainly from Imam al-Baqir and Imam al-Sadiq, peace bu upon them, like Tafsir al-al-'Ayyashi, Tafsir al-Qummi and other later works; although many ulama casted doubts on the authenticity of many of these reports, others have made use of them so we don't find unanimity in this regard. Other tafsirs focus on different qira'at, Arabic language, opinions of sahabah, tabi'in, other scholars and their reasonings: that is the case for the Tafsir of Shaykh Tusi and Allamah Tabrisi. Others emphasized the explanation of the Qur'an through the Qur'an, Quranic analysis,  intellectual and theological (and even philosophical) discussions; that is the case of Tafsir al-Mizan by 'Allamah Tabataba'i and those who have followed his methodology.

It is difficult to universally establish what is the"best shia tafsir" because it depends on the conclusions of singular scholars, students and researchers, and many times personal tendencies and "taste" play also a role in it. 

With prayers for your success.