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.
not able to be reached or achieved.
not able to occur, exist, or be done.
impossible in practice to do or carry out.
Transcendent was already mentioned (a great option if the intent is to be philosophical or metaphorical)
so good or unusual as to be impossible to copy; unique.
without an equal in quality or extent; matchless.
having no match or equal; unrivaled.
having no parallel or equal; exceptional.
Source Material for all entries: Oxford Living Dictionaries