إنَّ أفْضَلَ الدُّعَاءِ: الحَمْدُ لِلَّهِ، وَ أفْضَلُ الذِّكْرِ: لَا إلَهَ إلَّا اللهُ

Indeed the best duʿā (is): “All praise is for Allāh.” and the best remembrance (is): “None has the right to be worshiped except Allāh.”

Reference: Indeed the best duʿā (is): “All praise is for Allāh.” and the best remembrance (is): “None has the right to be worshiped except Allāh.” Al-Tirmidhī (#3383, 5/462), Ibn Mājah (#3800, 2/1249), and al-Hākim (1/503) and he authenticated it and al-Dhahabī agreed with him. Also see _Ṣaḥīḥ al-Jāmiʿ_ (#1104, 1/362).