Valmiki Ramayana is called adi Kavyam, the Very First Kavyam,EPIC, written in Sanskrit Language ,
First Sage Narada comes to Valmiki aashram and tells Valmiki Maharshi '' Sankshetpa Ramayanam '' ( Ranayanam in brief) and invokes interest in Valmiki Maharshi to write the Epic, and Sri rama was ruling Ayodhya then.
But then after Brahma Deva appears before Valmiki Maharshi and grants a boon that in Dhyaanam Valmiki will be able to see clearly what happened , what is going to happen and the conversations amon all characters clearly and even he, Valmiki will be knowing what is the minds of the various persons who ever they may be.With this boon Brahma Deva asks Valmiki to Compose the Great Epic Ramayana.
So Valmiki wrote or composed what exactly happened.He did not mix any of his imaginations or comments.He composed it Yadha tadham ( As it is ) as it happened and so it is called ITIHAASAM ( IT HAPPENED THIS WAY)
Later the great Epic encouraged many poets and saints to write Ramayanams by their own versions as they felt through Sri Rama Bahkti.
Ranganatha Ramayanam, Molla Ramayanam, Bhaskara Ramayanam ,and Ramacharaita manas. Ramayana Kalpataruvu etc many many great Ramayanams came by the POWER of Valmili Ramayana.It is their Rama Bhakti that made them compose those beautiful versions with their own noble imaginations.
We can not and SHOULD NOT Judge which is better than which.They are not to be compared but read with devotion and try to seek a way to have Darshan of Sri SitaRama and Anjaneya.They are all INSPIRED KAAVYAMS.
Tulisidas`s Ramacharitamanas is said to have been composed as Shiva appeared in dream and told him.He deviated at several episodes ( Ahalya -Gowtama episode, Ramayana Vadha ) etc from Valmiki Ramayanam.But this is with Sheer Rama Bhakti and no other intention.
Who are we to judge those Great Souls who wrote Ramayana.All are good .It is enough we understand them properly .
Our duty is to read and understand.
But Valmiki Ramayana alone is referred as Standard reference by Scholars.