Here are some others depending on your intent. I'm providing all of these because it was not clear from your question if you wanted something philosophical or concrete.
unattainable
not able to be reached or achieved.
impossible
not able to occur, exist, or be done.
impracticable
impossible in practice to do or carry out.
Transcendent was already mentioned (a great option if the intent is to be philosophical or metaphorical)
inimitable
so good or unusual as to be impossible to copy; unique.
incomparable
without an equal in quality or extent; matchless.
nonpareil
having no match or equal; unrivaled.
unparalleled
having no parallel or equal; exceptional.
Source Material for all entries: Oxford Living Dictionaries