Ibn Majah

Benefits and Issues:
➊ All prescribed (masnun) remembrances (adhkar) are a source of mercy and blessing, but the reward and blessings of (la ilaha illallah) are the greatest of all.

➋ Praising Allah is also a supplication (dua), because a person performs good deeds and remembrance with the intention of reward, and in this way, he attains the desired (reward).

➌ Another meaning is that the most excellent supplication is Surah Al-Fatihah, which is described in the hadith (al-hamdu lillah). In it, there is praise of Allah as well as supplication for guidance, bounty, and help.

Explanation:
1: (La ilaha illallah) is the most excellent form of remembrance (dhikr) because it is the essence of monotheism (tawhid), and there is no virtue equal to tawhid.
Furthermore, this phrase most strongly negates polytheism (shirk),
and both of these matters are the most beloved to Allah.
And (alhamdulillah) is the most excellent supplication (du'a) in the sense that when a servant praises Allah, he is expressing gratitude for Allah’s blessings,
and Allah the Exalted has said: “If you are grateful, I will surely increase you [in favor]” (If you give thanks, I will surely give you more), so what supplication (du'a) could be better than this request?
And some scholars have said that (alhamdulillah) is an allusion to the supplication found in Surah al-Hamd (al-Fatiha),
that is, “Ihdina al-sirat al-mustaqim” (Guide us to the straight path), and this is the most excellent supplication.