Reference : Hisn al-Muslim 264

إِنَّ أَفْضَلَ الدُّعَاءِ (الْحَمْدُ لِلَّهِ)، وَأَفْضَلَ الذِّكْرِ (لاَ إِلَهَ إِلاَّ اللَّهُ)
The most excellent invocation is:
Alḥamdulillāh

and the most excellent words of remembrance are:
Lā ilāha illallāh.

The most excellent invocation is:

Praise is for Allah.

and the most excellent words of remembrance are:

There is none worthy of worship but Allah.

Reference:
At-Tirmidhi 5/462, Ibn Majah 2/1249, and Al-Hakim who graded it authentic 
and Ath-Tbahabi agreed 1/503. See Al-Albani, Sahihul-Jami' As-Saghir 1/362.

Chapter: Excellence of Tasbih, Tahmid, Tahlil, and Takbir