Assalaam Aleikum Warahmatullahi WabarakatuhuWhich is the best dhikr with high/ heavy rewards.Jazaka Allah Kheir

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The best type of Thikr (mention of Allah) and the greatest in reward is the recitation of the Quran, as the reciter gets ten rewards for every letter.

Al-Majmoo’ reads: "Shaykh Abu Haamid quoted while commenting on this, that Ash-Shaafi’i stated that the best type of Thikr is the recitation of the Quran." [End of quote]

However, if there is a particular reason for being engaged in a certain kind of Thikr, such as exalting the mention of the Prophet on Friday and the night preceding Friday (which is Thursday night), and repeating the words after the Mu'aththin (the one who calls the Aathaan), and the legislated mention of Allah after the obligatory prayer, then spending the time to say this kind of Thikr is better in this case.

Shaykh ibn 'Uthaymeen was asked: “Which is better: Thikr or the recitation of the Quran? He replied:

"In general, the Quran is better than Thikr; however, Thikr becomes better when there is a certain reason for reciting it: an example for this is: reciting the Thikr after the obligatory prayers is better in its time than the recitation of the Quran; also, responding to the Mu’aththin in its time (i.e. when he is calling the Aathaan) is better than the recitation of the Quran, and so forth." [End of quote]

Allah knows best.