All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
You bear no sin in describing the Prophet as the “Greatest Prophet", because this means that he was the greatest of all the prophets, may Allaah exalt the mention of them all. Of course, this is true since Prophet Muhammad was the best of all the prophets and is the master of humankind.
There is no doubt that Allaah The Almighty is the Greatest and that none of His creatures are like Him. Allaah The Almighty Says (what means): {There is nothing like unto Him, and He is The Hearing, The Seeing.} [Quran, 42:11]
However, sometimes the same attributes that Allaah The Almighty possesses are used in their linguistic sense, to describe some of His creatures in a limited way. For example, Allaah The Almighty Says (what means):
It is known that the Hearing, the Seeing and the Forbearing are some of the names of Allaah The Almighty which describe certain attributes, but Allaah The Almighty also describes humans to be as such, but not in the same way as He is.
Allaah knows best.