Answer

Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.

In the Name of Allah, the Most Gracious, the Most Merciful.

All praise and thanks are due to Allah, and may the peace and blessings of Allah be upon His Messenger. 

Dear questioner, thank you very much for raising this good question that reflects a good understanding and truthful desire to have a clearer view of the teachings of Islam. May Allah crown this truthful desire with His satisfaction and reward.

At the time of the iqamah there is no better dhikr than repeating the words of the iqamah, except when ‘Qad qaamatus-salah’ is said. In this case, one should say ‘Aqamah Allah wa -adamaha’.
In this regard, Sheikh Sayyed Sabiq states in his well-known book, Fiqh Us-Sunnah: 

It is preferred that the one who hears the iqamah should repeat the words, except when ‘Qad qaamatus-salah’ is said. In this case, he should say ‘Aqamah Allah wa -adamaha’ (Allah establishes it and makes it everlasting.) Some of the companions reported that when Bilal said this phrase, the Prophet (peace and blessings be upon him) would say, “Allah establishes it and makes it everlasting.”

Almighty Allah knows best.