Lord Voldemort

"I regret it."

—Lord Voldemort after he kills Severus Snape

In his days as a Death Eater, Snape was one of Lord Voldemort's most trusted and devoted servants. In 1980, Snape overheard part of a prophecy foretelling Voldemort's downfall. After being caught, Snape hurried to tell Voldemort what he had heard, unaware that his information was incomplete. However, he turned against the Dark Lord the moment he decided to target Lily Potter.
When Voldemort returned to physical form in 1995, Snape ostensibly returned to him as a spy for the Order. Using Occlumency, Snape was able to close his mind, hide his true allegiance and maintain his cover as a faithful Death Eater. Snape was one of the rare Death Eaters to have the Dark Lord's complete trust and, after he murdered Albus Dumbledore, he became Voldemort's most trusted adviser. He was held in such high regard that Voldemort accepted to make him Headmaster of Hogwarts after seizing power.
However, Voldemort coldly had his pet snake, Nagini, kill Snape in 1998, believing it would make him the true master of the Elder Wand. Although he claimed he regretted its necessity, he felt no grief over murdering a Death Eater he considered one of his most trusted and useful servants. After Snape's death, Harry Potter revealed to Voldemort that Snape had managed to deceive him for years, and that his true loyalties had lain with Dumbledore since 1980, being dictated by his immense love for Lily Potter.
It is possible the reason Voldemort put so much trust in Snape is because they had a similar background; both half-bloods with a muggle father who resented or neglected them and saw him as a trusted Death Eater who shared the same hatred for for his father as he had for his own. Another possible reason was the fact that both hated Harry Potter for personal, albeit different, reasons.
Snape possessed the ability of flight which, we must assume, was taught to him by Lord Voldemort as he was shown possessing the same ability during the Battle of the Seven Potters. This shows that Lord Voldemort had a great deal of trust in Snape as he taught him a very unique ability, something he would never dare to do with any Death Eater.