* Neimaginebla: This is the most direct translation, literally meaning "not imaginable."
* Neimagineble: This is the adjective form, meaning "unimaginably."
* Senimagina: This is a more poetic translation, literally meaning "without imagination."
* Nerealigita: This means "unrealized" and could be used if you want to emphasize the impossibility of the thing.
The best translation will depend on the context of your sentence.