Tafsir of Al-Quranul Kareem to be exact. Why?
Because its the primary source of hidayah. Its the words of Allah. Imagine reading a reminder from ur big boss. Well Al-Quran is remider from our creator. Allah. The one and only. And learning tafsir is different from just reading the translation. Because sometimes u might misunderstood the meaning or failed to grasp the overall picture. Learning tafsir with proper guidance from ustaz or ustazah, u'll see that even one ayat (sentence) could take an hour or more of explanation to really understand its meaning. For example, our ustaz uses Tafsir Ibn Kathir (ok i need to ref this later coz i might miss-spell his name) and to explain the message contained in just one ayat, 2-3 hadiths or more would be refs plus another ayat from other surah will be linked to strengthen the message. Well it sound complicated, but trust me its soo worth it. Its exciting and fun and mind blowing and terrifying at once. And I thank Allah for the chance He provided me.
Start now. DO NOT miss the chance.