I have a very particular question about Me’raaj. Particularly, a verse in Surah Najm (verse 18) states that "He certainly saw of his Lord’s greatest sign"

Now my question is that regarding the Ayatil Kubra in this verse, I have heard two narrations – One states that Imam Amirul Mu’mineen says that I am the greatest sign of Allah and the Second states that Sayyeda Fatima alayhasalam is the greatest sign of Allah. So who did the Prophet (PBUH&HF) see at the highest point of his Me’raaj journey?

Everything he saw was some of Allah’s great signs. The whole meraj was a great sign. Hence although what you mentioned can be accepted from narrations, it’s not confined to one or two things only but a number of things, and Allah knows best

Holy Quran 53:18
لَقَدْ رَأَىٰ مِنْ آيَاتِ رَبِّهِ الْكُبْرَىٰ

Certainly he saw some of the greatest signs of his Lord.

The ‘min’ in the verse means some , and points to a number of signs