What was the essential message of all the Prophets?

The essential message of all the Prophets was one and the same, as Allah (سبحانه و تعالى) mentioned in the Qur'an:

“Verily, We have sent to every nation a messenger (saying), ‘Worship Allah and avoid false gods.” [1]

There was no difference in their basic messages also because the purpose of creation has only been one; to worship Allah. Allah (سبحانه و تعالى) mentioned in the Qur’an:

“I did not create the jinn and mankind except for My worship.” [2]

References:

[1] [Qur’an 16:36]

[2] [Qur’an 51:56]