I have a very particular question about Me’raaj that has been bothering me a lot. 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?

Some scholars state it is Imam Ali (a) due to the hadith you cited. Some say it was Jibr’eel. Some say he was aspects of Allah’s glory that we do not know and cannot know.
Allah knows best