The best remembrance
أَفْضَلُ الذِّكْرِ لَا إِلَهَ إِلَّا اللَّهُ
Afdalu-dh-dhikri la ilaha illa-llah.
"The best remembrance is: La ilaha illa Allah."
Narrated by Jabir ibn Abdullah. Sunan at-Tirmidhi 3383, Sunan Ibn Majah 3800
A short reflection
The shahada is also the highest dhikr. Repeat it as a sentence, not just as a certificate of entry into Islam.