Nanamika said: Originally Miku, but the song Nyan Cat uses itself is an UTAU rendition.
Seriously?! Alright, do you prefer "Miku Miku Ni Shite Ageru" or "Nyanyanyanyanyanyanya!" because "Miku Miku Ni Shite Ageru" is a better song to put into craziness... and it's a good one... you should try: