The Greatest Form of Dhikr

The angels remember Allah (dhikr) and the bearers of the Throne glorify Him, may He be exalted. They glorify Allah constantly, without ceasing, either at night or during the day. Because they glorify Allah so much, they are indeed those who truly glorify Allah's praises, and they have the right to be proud of that.

الَّذِينَ يَحْمِلُونَ الْعَرْشَ وَمَنْ حَوْلَهُ يُسَبِّحُونَ بِحَمْدِ رَبِّهِمْ وَيُؤْمِنُونَ بِهِ وَيَسْتَغْفِرُونَ لِلَّذِينَ آمَنُوا رَبَّنَا وَسِعْتَ كُلَّ شَيْءٍ رَحْمَةً وَعِلْمًا فَاغْفِرْ لِلَّذِينَ تَابُوا وَاتَّبَعُوا سَبِيلَكَ وَقِهِمْ عَذَابَ الْجَحِيمِ

Those angels who bear the Throne of Allah and those who stand around it glorify their Rabb with His praises, believe in Him and implore forgiveness for the believers, saying: "Our Rabb! You embrace all things with your mercy and knowledge. Forgive those who repent and follow Your Way, and save them from the punishment of the blazing fire.(Qur'an 40:7)

They glorify Allah so much because Tasbeeh is the greatest form of dhikr. 

Muslim narrated in his Saheeh that Abu Dharr said: "The Messenger of Allah (pbuh) was asked what kind of dhikr is the best. He said: "That which Allah has chosen for His angels or His slaves:

سُبْحَانَ اللّهِ وَ بِحَمْدِهِ 

(Subhaan Allaahi wa bi hamdihi)

Glory and praise be to Allah."

Muslim, 4/2093 hadith no. 2731.

(Taken from: The World of the Noble Angels, by Dr. Úmar S al-Ashqar - Islamic Creed Series book 2)