"Supreme Manifestation" is used to refer to Baha'u'llah as well as other Manifestations in the Writings. I did a search in Ocean and came across many examples of the use of this phrase. For instance,
"The Prophets of God, the supreme Manifestations, are like skilled physicians, and the contingent world is like the body of man: the divine laws are the remedy and treatment.
(Abdu'l-Baha, Some Answered Questions, p. 157)
I could and will if you like give you many more examples of the usage of "Supreme Manifestation" to refer to all, any or a particular Manifestation. We would never exalt one Manifestation over another. We do, though, recognize the importance of the Message delivered by Baha'u'llah and see it as the most recent direction from God. We also recognize His station as the Unifier of all religions, though many people at this early stage of the Baha'i Era, cannot recognize this and continue to try to divide the world by competing for peoples souls claiming it is the "will of God".
If you called Baha'u'llah the "Supreme Manifestation", you would be telling the truth. If you called Jesus of Nazereth the "Supreme Manifestation", you would also be telling the truth. If you said one was more important or more "supreme" than the other, you would be gravely mistaken.
Mick