Lord Voldemort has no Patronus. It is believed that since Lord Voldemort had already ripped his soul apart several times, there was no way the Dementors could sense anything remotely resembling a soul in him. Furthermore, it may also be the case that he has never really had a happy memory, at least not the type that is essential to conjure a Patronus. It is not known if Tom Riddle had a Patronus either but it is possible that there was never a need to find out as conjuring Patronuses were not as common in the school curriculum.