Shaykhul-Islaam Ibn Taymiyyah said:
“For Tawheed removes the essence of Shirk and al-Istighfaar erases its branches. Therefore, the most lofty praise is saying: لا اله الا الله (Laa ilaaha illallaah) and the most lofty du’aa is saying: أستغفر الله (astaghfiru-llaah).”
[Majmoo’ al-Fataawaa (11/697)]